Power storage device, method for manufacturing power storage device, and electronic device

ABSTRACT

To provide a power storage device whose charge and discharge characteristics are unlikely to be degraded by heat treatment. To provide a power storage device that is highly safe against heat treatment. The power storage device includes a positive electrode, a negative electrode, a separator, an electrolytic solution, and an exterior body. The separator is located between the positive electrode and the negative electrode. The separator contains polyphenylene sulfide or solvent-spun regenerated cellulosic fiber. The electrolytic solution contains a solute and two or more kinds of solvents. The solute contains LiBETA. One of the solvents is propylene carbonate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention relate to a power storage deviceand an electronic device.

Note that one embodiment of the present invention is not limited to theabove technical field. One embodiment of the invention disclosed in thisspecification and the like relates to an object, a method, or amanufacturing method. One embodiment of the present invention relates toa process, a machine, manufacture, or a composition of matter.Specifically, examples of the technical field of one embodiment of theinvention disclosed in this specification include a semiconductordevice, a display device, a light-emitting device, a power storagedevice, a memory device, an imaging device, a driving method thereof,and a manufacturing method thereof.

In this specification, the power storage device is a collective termdescribing elements and devices that have a power storage function. Forexample, a power storage device (also referred to as a secondarybattery) such as a lithium-ion secondary battery, a lithium-ioncapacitor, and an electric double layer capacitor are included in thecategory of the power storage device.

2. Description of the Related Art

In recent years, a variety of power storage devices, for example,lithium-ion secondary batteries, lithium-ion capacitors, and air cellshave been actively developed. In particular, demand for lithium-ionsecondary batteries with high output and high energy density has rapidlygrown with the development of the semiconductor industry, for portableinformation terminals such as mobile phones, smartphones, and laptopcomputers, portable music players, and digital cameras; medicalequipment; next-generation clean energy vehicles such as hybrid electricvehicles (HEV), electric vehicles (EV), and plug-in hybrid electricvehicles (PHEV); and the like. The lithium-ion secondary batteries areessential as rechargeable energy supply sources for today's informationsociety.

As described above, lithium-ion secondary batteries have been used for avariety of purposes in various fields. Properties necessary for suchlithium-ion secondary batteries are high energy density, excellent cycleperformance, safety in a variety of operation environments, and thelike.

A lithium-ion secondary battery includes at least a positive electrode,a negative electrode, and an electrolytic solution (Patent Document 1).

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2012-009418

SUMMARY OF THE INVENTION

Lithium-ion secondary batteries that are to be mounted in electronicdevices such as a wearable device and a portable information terminalneed to resist heat treatment performed when the electronic devices areprocessed. Particularly in the case where a housing of the electronicdevice and a lithium-ion secondary battery are integrally formed, thelithium-ion secondary battery needs to have heat resistance to atemperature higher than or equal to the manufacturing temperature of thehousing.

An object of one embodiment of the present invention is to provide apower storage device whose charging and discharging characteristics areunlikely to be degraded by heat treatment.

Another object of one embodiment of the present invention is to providea power storage device that is highly safe against heat treatment.

Another object of one embodiment of the present invention is to providea power storage device having high flexibility. Another object of oneembodiment of the present invention is to provide a novel power storagedevice, a novel electronic device, or the like.

Note that the description of these objects does not disturb theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Other objects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

One embodiment of the present invention is a power storage deviceincluding a positive electrode, a negative electrode, a separator, anelectrolytic solution, and an exterior body. The positive electrodeincludes a positive electrode active material layer and a positiveelectrode current collector. The negative electrode includes a negativeelectrode active material layer and a negative electrode currentcollector. The separator is located between the positive electrode andthe negative electrode. The separator contains polyphenylene sulfide orsolvent-spun regenerated cellulosic fiber. The electrolytic solutioncontains a solute and two or more kinds of solvents. The solute containslithium bis(pentafluoroethanesulfonyl)amide (LiBETA). One of thesolvents is propylene carbonate.

Another embodiment of the present invention is the power storage devicein which the solvents include propylene carbonate and ethylenecarbonate.

Another embodiment of the present invention is the power storage devicein which the negative electrode active material layer contains graphite.

Another embodiment of the present invention is the power storage devicein which the negative electrode active material layer includes sphericalnatural graphite. The spherical natural graphite includes a first regionand a second region. The first region covers the second region. Thefirst region has lower crystallinity than the second region.

Another embodiment of the present invention is the power storage devicein which the positive electrode active material layer contains LiCoO₂.

Another embodiment of the present invention is the power storage devicein which the positive electrode current collector contains aluminum orstainless steel.

Another embodiment of the present invention is a method formanufacturing the power storage device. In the method, a heating step isperformed at a first temperature for 10 minutes before energization ofthe power storage device. The first temperature is higher than or equalto 110° C. and lower than or equal to 190° C.

Another embodiment of the present invention is an electronic deviceincluding the power storage device, a band, a display panel, and ahousing. The power storage device includes a positive electrode lead anda negative electrode lead. The positive electrode lead is electricallyconnected to the positive electrode. The negative electrode lead iselectrically connected to the negative electrode. The power storagedevice is buried in the band. Part of the positive electrode lead andpart of the negative electrode lead protrude from the band. The powerstorage device has flexibility. The power storage device is electricallyconnected to the display panel. The display panel is included in thehousing. The band is connected to the housing. The band includes arubber material.

Another embodiment of the present invention is the electronic device inwhich the rubber material is fluorine rubber or silicone rubber.

One embodiment of the present invention can provide a power storagedevice whose charging and discharging characteristics are unlikely to bedegraded by heat treatment.

One embodiment of the present invention can provide a power storagedevice that is highly safe against heat treatment.

One embodiment of the present invention can provide a power storagedevice having high flexibility. One embodiment of the present inventioncan provide a novel power storage device, a novel electronic device, orthe like.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily have all the effects listed above. Other effects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C illustrate an example of a power storage device andexamples of electrodes.

FIGS. 2A and 2B each illustrate an example of a power storage device.

FIGS. 3A and 3B each illustrate an example of a power storage device.

FIGS. 4A and 4B each illustrate an example of a power storage device.

FIGS. 5A to 5F illustrate examples of embossing.

FIGS. 6A and 6B illustrate an example of a power storage device.

FIG. 7 illustrates an example of a power storage device.

FIG. 8 illustrates an example of a power storage device.

FIGS. 9A to 9C illustrate an example of a method for manufacturing apower storage device.

FIGS. 10A to 10C illustrate an example of a method for manufacturing apower storage device.

FIG. 11 illustrates an example of a method for manufacturing a powerstorage device.

FIGS. 12A to 12C illustrate examples of an electronic device, a band,and a power storage device.

FIGS. 13A to 13C illustrate examples of a band and power storagedevices.

FIGS. 14A and 14B illustrate an example of a power storage device.

FIG. 15A to 15C illustrate an example of a method for detecting leakage.

FIGS. 16A and 16B illustrate an example of a power storage device.

FIGS. 17A and 17B illustrate an example of a power storage device.

FIG. 18 illustrates an example of a power storage device.

FIGS. 19A to 19D illustrate an example of a method for fabricating apower storage device.

FIGS. 20A, 20B, 20C1, and 20C2 illustrate an example of a power storagedevice.

FIG. 21 illustrates an example of a power storage device.

FIGS. 22A to 22D illustrate an example of a method for fabricating apower storage device.

FIG. 23 illustrates an example of a power storage device.

FIGS. 24A to 24F illustrate examples of electronic devices.

FIGS. 25A to 25D illustrate examples of electronic devices.

FIGS. 26A to 26C illustrate an example of an electronic device.

FIG. 27 illustrates examples of electronic devices.

FIGS. 28A and 28B illustrate examples of electronic devices.

FIGS. 29A to 29D each show charge and discharge curves in Example 1.

FIGS. 30A to 30D each show charge and discharge curves in Example 1.

FIGS. 31A to 31C are cross-sectional TEM images in Example 2.

FIG. 32 shows analysis results of Raman spectra in Example 2.

FIGS. 33A and 33B each show charge and discharge curves in Example 3.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments and an example will be described in detail with reference todrawings. Note that the present invention is not limited to thedescription below, and it is easily understood by those skilled in theart that various changes and modifications can be made without departingfrom the spirit and scope of the present invention. Accordingly, thepresent invention should not be interpreted as being limited to thecontent of the embodiments and example below.

Note that in the structures of the invention described below, the sameportions or portions having similar functions are denoted by the samereference numerals in different drawings, and the descriptions of suchportions are not repeated. Furthermore, the same hatching pattern isapplied to portions having similar functions, and the portions are notspecially denoted by reference numerals in some cases.

In addition, the position, size, range, or the like of each structureillustrated in drawings and the like is not accurately represented insome cases for easy understanding. Therefore, the disclosed invention isnot necessarily limited to the position, size, range, or the likedisclosed in the drawings and the like.

Note that the terms “film” and “layer” can be interchanged with eachother depending on the case or circumstances. For example, the term“conductive layer” can be changed into the term “conductive film” insome cases. Also, the term “insulating film” can be changed into theterm “insulating layer” in some cases.

Embodiment 1

In this embodiment, power storage devices of embodiments of the presentinvention will be described with reference to FIGS. 1A to 1C to FIG. 11.

The power storage device of one embodiment of the present inventionincludes a positive electrode, a negative electrode, a separator, anelectrolytic solution, and an exterior body.

Note that in this specification and the like, the electrolytic solutionis not limited to a liquid one and may be a gelled or solid one.

For high heat resistance of the power storage device, first, a solutecontained in the electrolytic solution needs to have high stability athigh temperature. For example, lithium hexafluorophosphate (LiPF₆),which is widely used as a lithium salt serving as a solute, isdecomposed into LiF and PF₅ at high temperature. It is said that PF₅causes the decomposition of a solvent; thus, LiPF₆ seems to have lowstability at high temperature for a solute.

In view of the above, as a solute in the electrolytic solution in thepower storage device of one embodiment of the present invention, lithiumbis(pentafluoroethanesulfonyl)amide (Li(C₂F₅SO₂)₂N, abbreviation:LiBETA) is preferably used. LiBETA has a decomposition temperature of350° C., and has high heat resistance. Furthermore, in the case where,for example, aluminum is used for a positive electrode currentcollector, the use of LiBETA can inhibit aluminum dissolution from thepositive electrode current collector because a passivating film iseasily formed on a surface of the positive electrode current collectorwhen the power storage device is charged and discharged.

Moreover, to increase the heat resistance of the power storage device, asolvent contained in the electrolytic solution preferably has a highboiling point and low vapor pressure. An example of a nonaqueous solventhaving a boiling point of 242° C. is propylene carbonate (PC).

However, in the case where graphite is used as a negative electrodeactive material, PC does not form a passivating film on a surface ofgraphite but is intercalated between graphite layers together withlithium ions, separating part of the graphite layers from a graphiteparticle in some cases.

Thus, the electrolytic solution in the power storage device of oneembodiment of the present invention contains two or more kinds ofsolvents, including at least PC. The solvent in the electrolyticsolution other than PC preferably has a function of forming apassivating film on a surface of the negative electrode. Examples of thesolvent contained in the electrolytic solution other than PC includeethylene carbonate (EC) and vinylene carbonate (VC).

The boiling point of EC is 248° C., and EC has high heat resistance andlow vapor pressure. Depending on a selected graphite material, a mixedsolvent of PC and EC can inhibit separation of a graphite layer. Forexample, a 1:1 (volume ratio) mixture of PC and EC can be used as thesolvent. In the case where graphite is used for the negative electrode,a graphite material in which PC is unlikely to be intercalated betweenlayers is preferably selected. In the power storage device of oneembodiment of the present invention, spherical natural graphite is usedfor the negative electrode active material. The spherical naturalgraphite includes a region having low crystallinity on the surface side,whereby PC intercalation between layers of the spherical naturalgraphite may be reduced.

It is confirmed that the following aluminum laminated cell does notexpand due to heat treatment performed at 170° C. for 15 minutes. In thealuminum laminated cell, encapsulated is an electrolytic solution inwhich 1 mol/l of LiBETA is dissolved and PC and EC are mixed at a volumeratio of 1:1. Thus, the solvent in which PC and EC are mixed at a volumeratio of 1:1 has high stability and low vapor pressure at hightemperature.

Polyethylene, polypropylene, and the like, which are generally used as aseparator, are sensitive to heat. Minute pores of a separator might beblocked at high temperature, resulting in malfunction of the powerstorage device.

In view of the above, in the power storage device of one embodiment ofthe present invention, a separator containing polyphenylene sulfide or aseparator containing solvent-spun regenerated cellulosic fiber is used.

The separator containing polyphenylene sulfide and the separatorcontaining solvent-spun regenerated cellulosic fiber have high heatresistance and high chemical resistance.

Moreover, the separator containing polyphenylene sulfide and theseparator containing solvent-spun regenerated cellulosic fiber have lowreactivity to the electrolytic solution at high temperature. Thus,degradation of the output characteristics and the charge and dischargecycle performance can be inhibited.

Structural Example of Power Storage Device

Next, a specific structure of the power storage device of one embodimentof the present invention will be described below.

FIG. 1A illustrates a power storage device 500, which is a power storagedevice of one embodiment of the present invention. Although FIG. 1Aillustrates a mode of a thin power storage device as an example of thepower storage device 500, one embodiment of the present invention is notlimited to this example.

As illustrated in FIG. 1A, the power storage device 500 includes apositive electrode 503, a negative electrode 506, a separator 507, andan exterior body 509. The power storage device 500 may include apositive electrode lead 510 and a negative electrode lead 511. A bondingportion 518 corresponds to a thermocompression bonding portion in theouter region of the exterior body 509.

FIGS. 2A and 2B each illustrate an example of a cross-sectional viewalong dashed-dotted line A1-A2 in FIG. 1A. FIGS. 2A and 2B eachillustrate a cross-sectional structure of the power storage device 500that is formed using a pair of the positive electrode 503 and thenegative electrode 506.

As illustrated in FIGS. 2A and 2B, the power storage device 500 includesthe positive electrode 503, the negative electrode 506, the separator507, an electrolytic solution 508, and the exterior bodies 509. Theseparator 507 is located between the positive electrode 503 and thenegative electrode 506. A space surrounded by the exterior bodies 509 isfilled with the electrolytic solution 508.

The positive electrode 503 includes a positive electrode active materiallayer 502 and a positive electrode current collector 501. The negativeelectrode 506 includes a negative electrode active material layer 505and a negative electrode current collector 504. The active materiallayer is formed on one surface or opposite surfaces of the currentcollector. The separator 507 is positioned between the positiveelectrode current collector 501 and the negative electrode currentcollector 504.

The power storage device includes one or more positive electrodes andone or more negative electrodes. For example, the power storage devicecan have a layered structure including a plurality of positiveelectrodes and a plurality of negative electrodes.

FIG. 3A illustrates another example of a cross-sectional view alongdashed-dotted line A1-A2 in FIG. 1A. FIG. 3B is a cross-sectional viewalong dashed-dotted line B1-B2 in FIG. 1A.

FIGS. 3A and 3B each illustrate a cross-sectional structure of the powerstorage device 500 that is formed using a plurality of pairs of thepositive and negative electrodes 503 and 506. There is no limitation onthe number of electrode layers of the power storage device 500. In thecase of using a large number of electrode layers, the power storagedevice can have high capacity. In contrast, in the case of using a smallnumber of electrode layers, the power storage device can have a smallthickness and high flexibility.

The examples in FIGS. 3A and 3B each include two positive electrodes 503in each of which the positive electrode active material layer 502 isprovided on one surface of the positive electrode current collector 501;two positive electrodes 503 in each of which the positive electrodeactive material layers 502 are provided on opposite surfaces of thepositive electrode current collector 501; and three negative electrodes506 in each of which the negative electrode active material layers 505are provided on opposite surfaces of the negative electrode currentcollector 504. In other words, the power storage device 500 includes sixpositive electrode active material layers 502 and six negative electrodeactive material layers 505. Note that although the separator 507 has abag-like shape in the examples illustrated in FIGS. 3A and 3B, thepresent invention is not limited to this example and the separator 507may have a strip shape or a bellows shape.

Alternatively, one positive electrode in which both surfaces of thepositive electrode current collector 501 are provided with the positiveelectrode active material layers 502 in FIGS. 3A and 3B is preferablyreplaced with two positive electrodes in each of which one surface ofthe positive electrode current collector 501 is provided with thepositive electrode active material layer 502. Similarly, one negativeelectrode in which both surfaces of the negative electrode currentcollector 504 are provided with the negative electrode active materiallayers 505 is preferably replaced with two negative electrodes in eachof which one surface of the negative electrode current collector 504 isprovided with the negative electrode active material layer 505. In thepower storage device 500 in FIGS. 4A and 4B, surfaces of the positiveelectrode current collectors 501 on the side not provided with thepositive electrode active material layer 502 face and are in contactwith each other, and surfaces of the negative electrode currentcollectors 504 on the side not provided with the negative electrodeactive material layer 505 face and are in contact with each other. Sucha structure allows the interface between the two positive electrodecurrent collectors 501 and the two negative electrode current collectors504 to serve as sliding planes when the power storage device 500 iscurved, relieving stress caused in the power storage device 500.

FIG. 1B illustrates the appearance of the positive electrode 503. Thepositive electrode 503 includes the positive electrode current collector501 and the positive electrode active material layer 502.

FIG. 1C illustrates the appearance of the negative electrode 506. Thenegative electrode 506 includes the negative electrode current collector504 and the negative electrode active material layer 505.

The positive electrode 503 and the negative electrode 506 preferablyinclude tab regions so that a plurality of stacked positive electrodescan be electrically connected to each other and a plurality of stackednegative electrodes can be electrically connected to each other.Furthermore, a lead is preferably electrically connected to the tabregion.

As illustrated in FIG. 1B, the positive electrode 503 preferablyincludes the tab region 281. The positive electrode lead 510 ispreferably welded to part of the tab region 281. The tab region 281preferably includes a region where the positive electrode currentcollector 501 is exposed. When the positive electrode lead 510 is weldedto the region where the positive electrode current collector 501 isexposed, contact resistance can be further reduced. Although FIG. 1Billustrates the example where the positive electrode current collector501 is exposed in the entire tab region 281, the tab region 281 maypartly include the positive electrode active material layer 502.

As illustrated in FIG. 1C, the negative electrode 506 preferablyincludes the tab region 282. The negative electrode lead 511 ispreferably welded to part of the tab region 282. The tab region 282preferably includes a region where the negative electrode currentcollector 504 is exposed. When the negative electrode lead 511 is weldedto the region where the negative electrode current collector 504 isexposed, contact resistance can be further reduced. Although FIG. 1Cillustrates the example where the negative electrode current collector504 is exposed in the entire tab region 282, the tab region 282 maypartly include the negative electrode active material layer 505.

Although FIG. 1A illustrates the example where the ends of the positiveelectrode 503 and the negative electrode 506 are substantially alignedwith each other, part of the positive electrode 503 may extend beyondthe end of the negative electrode 506.

In the power storage device 500, the area of a region where the negativeelectrode 506 does not overlap with the positive electrode 503 ispreferably as small as possible.

In the example illustrated in FIG. 2A, the end of the negative electrode506 is located inward from the end of the positive electrode 503. Withthis structure, the entire negative electrode 506 can overlap with thepositive electrode 503 or the area of the region where the negativeelectrode 506 does not overlap with the positive electrode 503 can besmall.

The areas of the positive electrode 503 and the negative electrode 506in the power storage device 500 are preferably substantially equal. Forexample, the areas of the positive electrode 503 and the negativeelectrode 506 that face each other with the separator 507 therebetweenare preferably substantially equal. For example, the areas of thepositive electrode active material layer 502 and the negative electrodeactive material layer 505 that face each other with the separator 507therebetween are preferably substantially equal.

For example, as illustrated in FIGS. 3A and 3B, the area of the positiveelectrode 503 on the separator 507 side is preferably substantiallyequal to the area of the negative electrode 506 on the separator 507side. When the area of a surface of the positive electrode 503 on thenegative electrode 506 side is substantially equal to the area of asurface of the negative electrode 506 on the positive electrode 503side, the region where the negative electrode 506 does not overlap withthe positive electrode 503 can be small (does not exist, ideally),whereby the power storage device 500 can have reduced irreversiblecapacity. Alternatively, as illustrated in FIGS. 3A and 3B, the area ofthe surface of the positive electrode active material layer 502 on theseparator 507 side is preferably substantially equal to the area of thesurface of the negative electrode active material layer 505 on theseparator 507 side.

As illustrated in FIGS. 3A and 3B, the end of the positive electrode 503and the end of the negative electrode 506 are preferably substantiallyaligned with each other. Ends of the positive electrode active materiallayer 502 and the negative electrode active material layer 505 arepreferably substantially aligned with each other.

In the example illustrated in FIG. 2B, the end of the positive electrode503 is located inward from the end of the negative electrode 506. Withthis structure, the entire positive electrode 503 can overlap with thenegative electrode 506 or the area of the region where the positiveelectrode 503 does not overlap with the negative electrode 506 can besmall. In the case where the end of the negative electrode 506 islocated inward from the end of the positive electrode 503, a currentsometimes concentrates at the end portion of the negative electrode 506.For example, concentration of a current in part of the negativeelectrode 506 results in deposition of lithium on the negative electrode506 in some cases. By reducing the area of the region where the positiveelectrode 503 does not overlap with the negative electrode 506,concentration of a current in part of the negative electrode 506 can beinhibited. As a result, for example, deposition of lithium on thenegative electrode 506 can be inhibited, which is preferable.

As illustrated in FIG. 1A, the positive electrode lead 510 is preferablyelectrically connected to the positive electrode 503. Similarly, thenegative electrode lead 511 is preferably electrically connected to thenegative electrode 506. The positive electrode lead 510 and the negativeelectrode lead 511 are exposed to the outside of the exterior body 509so as to serve as terminals for electrical contact with an externalportion.

The positive electrode current collector 501 and the negative electrodecurrent collector 504 can double as terminals for electrical contactwith an external portion. In that case, the positive electrode currentcollector 501 and the negative electrode current collector 504 may bearranged such that part of the positive electrode current collector 501and part of the negative electrode current collector 504 are exposed tothe outside of the exterior body 509 without using leads.

Note that part of a surface of the exterior body 509 preferably hasprojections and depressions. This can relieve stress applied to theexterior body 509 when the power storage device 500 is curved. Thus, thepower storage device 500 can have high flexibility. Such projections anddepressions can be formed by embossing the exterior body 509 before thepower storage device 500 is assembled.

Here, embossing, which is a kind of pressing, will be described.

FIGS. 5A to 5F are cross-sectional views illustrating examples ofembossing. Note that embossing refers to processing for formingunevenness on a film by bringing an embossing roll whose surface hasunevenness into contact with the film with pressure. Note that theembossing roll is a roll whose surface is patterned.

FIG. 5A illustrates an example where one surface of a film 50 used forthe exterior body 509 is embossed.

FIG. 5A illustrates the state where a film 50 is sandwiched between anembossing roll 53 in contact with the one surface of the film and a roll54 in contact with the other surface and the film 50 is transferred in adirection 60. The surface of the film is patterned by pressure or heat.

Processing illustrated in FIG. 5A is called one-side embossing, whichcan be performed by a combination of the embossing roll 53 and the roll54 (a metal roll or an elastic roll such as a rubber roll).

FIG. 5B illustrates the state where a film 51 whose one surface isembossed is sandwiched between the embossing roll 53 and the roll 54 andis transferred in the direction 60. The embossing roll 53 rolls along anon-embossed surface of the film 51; thus, both surfaces of the film 51are embossed. As described here, one film can be embossed more thanonce.

FIG. 5C is an enlarged view of a cross section of a film 52 whose bothsurfaces are embossed. Note that H₁ represents the thickness of the filmin depressions or projections, and H₂ represents the thickness of thefilm at a boundary portion between a depression and its adjacentprojection or the thickness of the film at a boundary portion between aprojection and its adjacent depression. The thickness of the film is notuniform, and H₂ is smaller than H₁.

FIG. 5D illustrates another example where both surfaces of a film areembossed.

FIG. 5D illustrates the state where the film 50 is sandwiched betweenthe embossing roll 53 in contact with one surface of the film and anembossing roll 55 in contact with the other surface and the film 50 isbeing transferred in the direction 60.

FIG. 5D illustrates a combination of the embossing roll 53 and theembossing roll 55, which are a couple of embossing rolls. The surface ofthe film 50 is patterned by alternately provided projections anddepressions for embossing and debossing part of the surface of the film50.

FIG. 5E illustrates the case of using the embossing roll 53 and anembossing roll 56 whose protrusions have a pitch different from that ofprotrusions of the embossing roll 55 in FIG. 5D. Note that a protrusionpitch or an embossing pitch is the distance between the tops of adjacentprotrusions. For example, a distance P in FIG. 5E is a protrusion pitchor an embossing pitch. FIG. 5E illustrates the state where the film 50is sandwiched between the embossing roll 53 and the embossing roll 56and is transferred in the direction 60. The film processed using theembossing rolls with different protrusion pitches can have surfaces withdifferent embossing pitches.

FIG. 5F illustrates the state where the film 50 is sandwiched between anembossing roll 57 in contact with one surface of the film and anembossing roll 58 in contact with the other surface and the film 50 istransferred in the direction 60.

Processing illustrated in FIG. 5F is called tip-to-tip both-sideembossing performed by a combination of the embossing roll 57 and theembossing roll 58 that has the same pattern as the embossing roll 57.The phases of the projections and depressions of the two embossing rollsare coordinate, so that substantially the same pattern can be formed onboth surfaces of the film 50. Unlike in the case of FIG. 5F, embossingmay be performed without coordinating the phases of the projections anddepressions of the same embossing rolls.

An embossing plate can be used instead of the embossing roll.Furthermore, embossing is not necessarily employed, and any method thatallows formation of a relief on part of the film can be employed.

FIG. 6A illustrates an example of the power storage device 500 using anexterior body 529 having projections and depressions formed by theembossing described above. FIG. 6B is a cross-sectional view taken alongthe dashed-dotted line H1-H2 in FIG. 6A. The structure of FIG. 6Bwithout the exterior body 529 is similar to the structure of FIG. 3B.

The projections and depressions of the exterior body 529 are formed soas to include a region overlapping with the positive electrode 503 andthe negative electrode 506. In FIG. 6A, the bonding portion 518 does nothave projections and depressions, but may have projections anddepressions.

Furthermore, the projections and depressions of the exterior body 529are formed at regular intervals in the long axis direction of the powerstorage device 500 (the Y direction in FIG. 6A). In other words, onedepression and one projection are formed so as to extend in the shortaxis direction of the power storage device 500 (the X direction in FIG.6A). Such projections and depressions can relieve stress applied whenthe power storage device 500 is curved in the long axis direction.

Note that the projections and depressions of the exterior body 529 maybe formed so as to have a geometric pattern in which diagonal lines intwo directions cross each other (see FIG. 7). Such projections anddepressions can relieve stresses caused by curving the power storagedevice 500 in at least two directions.

Although the positive electrode lead 510 and the negative electrode lead511 are provided on the same side of the power storage device 500 inFIG. 1A, the positive electrode lead 510 and the negative electrode lead511 may be provided on different sides of the power storage device 500as illustrated in FIG. 8. The leads of the power storage device of oneembodiment of the present invention can be freely positioned asdescribed above; therefore, the degree of freedom in design is high.Accordingly, a product including the power storage device of oneembodiment of the present invention can have a high degree of freedom indesign. Furthermore, a yield of products each including the powerstorage device of one embodiment of the present invention can beincreased.

<Example of Manufacturing Method for Power Storage Device>

Next, an example of a manufacturing method for the power storage device500, which is a power storage device of one embodiment of the presentinvention, will be described with reference to FIGS. 9A and 9B to FIG.11.

First, the positive electrode 503, the negative electrode 506, and theseparator 507 are stacked. Specifically, the separator 507 is positionedover the positive electrode 503. Then, the negative electrode 506 ispositioned over the separator 507. In the case of using two or morepositive electrode-negative electrode pairs, another separator 507 ispositioned over the negative electrode 506, and then, the positiveelectrode 503 is positioned. In this manner, the positive electrodes 503and the negative electrodes 506 are alternately stacked and separated bythe separator 507.

Alternatively, the separator 507 may have a bag-like shape. Theelectrode is preferably surrounded by the separator 507, in which casethe electrode is less likely to be damaged during a fabricating process.

First, the positive electrode 503 is positioned over the separator 507.Then, the separator 507 is folded along a broken line in FIG. 9A so thatthe positive electrode 503 is sandwiched by the separator 507. Althoughthe example where the positive electrode 503 is sandwiched by theseparator 507 is described here, the negative electrode 506 may besandwiched by the separator 507.

Here, the outer edges of the separator 507 outside the positiveelectrode 503 are bonded so that the separator 507 has a bag-like shape(or an envelope-like shape). The bonding of the outer edges of theseparator 507 can be performed with the use of an adhesive or the like,by ultrasonic welding, or by thermal fusion bonding.

Next, the outer edges of the separator 507 are bonded by heating.Bonding portions 514 are illustrated in FIG. 9A. In such a manner, thepositive electrode 503 can be covered with the separator 507.

Note that in the case where a material such as cellulose or paper isused as the separator 507, the outer edges of the separator 507 arebonded using an adhesive or the like. The amount of the adhesive ispreferably small. The outer edges of the separator 507 are bonded suchthat an electrode (the positive electrode 503 in FIG. 9A) sandwichedbetween facing portions of the separator 507 does not protrude from theseparator 507; thus, for example, when the bonding portions 514 areformed as illustrated in FIG. 9B, the amount of the adhesive can bereduced. In FIG. 9B, the bonding portions 514 are formed at thefollowing portions of the outer edges of the separator 507: portions oftwo sides intersecting with a side where a fold is formed that are closeto the fold; and a portion of a side opposite to the side where the foldis formed.

Then, the positive electrodes 503 each covered with the separator 507and the negative electrodes 506 are alternately stacked as illustratedin FIG. 9C. Furthermore, the positive electrode lead 510 and thenegative electrode lead 511 each having a sealing layer 115 areprepared.

After that, the positive electrode lead 510 having the sealing layer 115is connected to the tab region 281 of the positive electrode 503 asillustrated in FIG. 10A. FIG. 10B is an enlarged view of a connectionportion. The tab region 281 of the positive electrode 503 and thepositive electrode lead 510 are electrically connected to each other byirradiating the bonding portion 512 with ultrasonic waves while applyingpressure thereto (ultrasonic welding). In that case, a curved portion513 is preferably provided in the tab region 281.

This curved portion 513 can relieve stress due to external force appliedafter fabrication of the power storage device 500. Thus, the powerstorage device 500 can have high reliability.

The negative electrode lead 511 can be electrically connected to the tabregion 282 of the negative electrode 506 by a similar method.

Subsequently, the positive electrode 503, the negative electrode 506,and the separator 507 are positioned over an exterior body 509.

Then, the exterior body 509 is folded along a portion shown by a dottedline in the vicinity of a center portion of the exterior body 509 inFIG. 10C.

In FIG. 11, the thermocompression bonding portion in the outer edges ofthe exterior body 509 is illustrated as a bonding portion 118. The outeredges of the exterior body 509 except an inlet 119 for introducing theelectrolytic solution 508 are bonded by thermocompression bonding. Inthermocompression bonding, sealing layers provided over the leads arealso melted, thereby fixing the leads and the exterior body 509 to eachother. Moreover, adhesion between the exterior body 509 and the leadscan be increased.

After that, in a reduced-pressure atmosphere or an inert gas atmosphere,a desired amount of electrolytic solution 508 is introduced to theinside of the exterior body 509 from the inlet 119. Lastly, the inlet119 is sealed by thermocompression bonding. Through the above steps, thepower storage device 500, which is a thin power storage device, can befabricated.

Aging may be performed after fabrication of the power storage device500. The aging can be performed under the following conditions, forexample. Charging is performed at a rate of 0.001 C or more and 0.2 C orless at temperatures higher than or equal to room temperature and lowerthan or equal to 50° C. In the case where an electrolytic solution isdecomposed and a gas is generated and accumulates between theelectrodes, the electrolytic solution cannot be in contact with asurface of the electrode in some regions. That is to say, an effectualreaction area of the electrode is reduced and effectual resistance isincreased.

When the resistance is extremely increased, the negative electrodepotential is decreased. Consequently, lithium is intercalated intographite and lithium is deposited on the surface of graphite. Thelithium deposition might reduce capacity. For example, if a coating filmor the like is grown on the surface after lithium deposition, lithiumdeposited on the surface cannot be dissolved again. This lithium cannotcontribute to capacity. In addition, when deposited lithium isphysically collapsed and conduction with the electrode is lost, thelithium also cannot contribute to capacity. Therefore, the gas ispreferably released to prevent the potential of the negative electrodefrom reaching the potential of lithium because of an increase in acharging voltage.

In the case of performing degasification, for example, part of theexterior body of the thin power storage device is cut to open the powerstorage device. When the exterior body is expanded because of a gas, theform of the exterior body is preferably adjusted. Furthermore, theelectrolytic solution may be added as needed before resealing. In thecase where degasification cannot be performed, a space for releasing agas may be provided in the cell so that a gas that accumulates betweenthe electrodes can be released from between the electrodes.Alternatively, a space formed by the use of the embossed laminateexterior body described above can be utilized as a space for releasing agas.

After the release of the gas, the charging state may be maintained attemperatures higher than room temperature, preferably higher than orequal to 30° C. and lower than or equal to 60° C., more preferablyhigher than or equal to 35° C. and lower than or equal to 50° C. for,for example, 1 hour or more and 100 hours or less. In the initialcharging, an electrolytic solution decomposed on the surface forms acoating film. The formed coating film may thus be densified when thecharging state is held at temperatures higher than room temperatureafter the release of the gas, for example.

<Components of Power Storage Device>

Components of the power storage device of one embodiment of the presentinvention will be described in detail below. When a flexible material isselected from materials of the members described in this embodiment andused, a flexible power storage device can be fabricated.

<<Separator>>

In the power storage device of one embodiment of the present invention,a separator containing polyphenylene sulfide or solvent-spun regeneratedcellulosic fiber is used. The separator can have either a single-layerstructure or a layered structure, and may have a layered structure of aseparator containing solvent-spun regenerated cellulosic fiber andanother separator, for example.

As a material for the separator, one or more materials selected from thefollowing can be used besides polyphenylene sulfide and solvent-spunregenerated cellulosic fiber: polypropylene sulfide, a fluorine-basedpolymer, cellulose, paper, nonwoven fabric, glass fiber, ceramics,synthetic fiber such as nylon (polyamide), vinylon (polyvinyl alcoholfiber), polyester, acrylic, polyolefin, or polyurethane, and the like.

<<Electrolytic Solution>>

The electrolytic solution contains an electrolyte and a solvent. Notethat in this specification and the like, an electrolyte is referred toas a solute in some cases.

As the solvent of the electrolytic solution, a material with carrier ionmobility is used. In particular, the solvent preferably has high heatresistance and low reactivity to a graphite negative electrode. In thepower storage device of one embodiment of the present invention, amixture of PC and EC is used as the solvent.

As the solvent, an aprotic organic solvent is preferably used. Forexample, one of EC, PC, butylene carbonate, γ-butyrolactone,γ-valerolactone, dimethyl sulfoxide, methyl diglyme, benzonitrile, andsulfolane can be used, or two or more of these solvents can be used inan appropriate combination in an appropriate ratio.

When a gelled high-molecular material is used as the solvent of theelectrolytic solution, safety against liquid leakage and the like isimproved. Furthermore, the power storage device can be thinner and morelightweight. Typical examples of gelled high-molecular materials includea silicone gel, an acrylic gel, an acrylonitrile gel, a polyethyleneoxide-based gel, a polypropylene oxide-based gel, a fluorine-basedpolymer gel, and the like.

Alternatively, the use of one or more kinds of ionic liquids (roomtemperature molten salts) which have features of non-flammability andnon-volatility as a solvent of the electrolytic solution can prevent thepower storage device from exploding or catching fire even when the powerstorage device internally shorts out or the internal temperatureincreases owing to overcharging and others. Thus, the power storagedevice has improved safety.

As the solute, a material that has carrier ion mobility and containscarrier ions can be used. In the case where carrier ions are lithiumions, the solute is a lithium salt. As a lithium salt, LiBETA, lithiumbis(trifluoromethanesulfonyl)amide (Li(CF₃SO₂)₂N, abbreviation: LiTFSA),lithium bis(fluorosulfonyl)amide (Li(FSO₂)₂N, abbreviation: LiFSA),LiBF₄, lithium bis(oxalato)borate (LiB(C₂O₄)₂, abbreviation: LiBOB), orthe like, which has high heat resistance, is preferably used.

In the power storage device, when a metal included in the positiveelectrode current collector is dissolved by a battery reaction betweenthe electrolytic solution and the current collector, the capacity of thepower storage device is decreased and the power storage devicedeteriorates. That is, the capacity is significantly decreased ascharging and discharging are repeated through the cycle performance testof the power storage device, and the lifetime of the power storagedevice becomes short. Furthermore, when metal dissolution from thecurrent collector at a connection portion between the lead and thecurrent collector proceeds, disconnection might occur. In one embodimentof the present invention, a material which is unlikely to react with thecurrent collector and thus is unlikely to cause the dissolution of themetal in the current collector is used for the solute material containedin the electrolytic solution.

Examples of a metal in materials for the positive electrode currentcollector include aluminum and stainless steel. In one embodiment of thepresent invention, for a solute material used for the electrolyticsolution, the solute that is unlikely to dissolve such a metal includedin the positive electrode current collector is used. Specifically,LiBETA can be given as a lithium salt that can be used as the solute inone embodiment of the present invention.

Therefore, in the power storage device of one embodiment of the presentinvention, the dissolution of the metal included in the positiveelectrode current collector into the electrolytic solution is inhibited,so that the deterioration of the positive electrode current collector isinhibited. In addition, the deposition of the metal on a surface of thenegative electrode is inhibited, so that the capacity reduction issmall, and the power storage device can have a favorable cycle lifetime.

Other than the above electrolytes, one of lithium salts such as LiPF₆,LiClO₄, LiAsF₆, LiAlCl₄, LiSCN, LiBr, LiI, Li₂SO₄, Li₂B₁₀Cl₁₀,Li₂B₁₂Cl₁₂, LiCF₃SO₃, LiC₄F₉SO₃, LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃,LiN(C₄F₉SO₂)(CF₃SO₂), and LiN(C₂F₅SO₂)₂ can be used, or two or more ofthese lithium salts can be used in an appropriate combination in anappropriate ratio.

Although the case where carrier ions are lithium ions in the aboveelectrolyte is described, carrier ions other than lithium ions can beused. When the carrier ions other than lithium ions are alkali metalions or alkaline-earth metal ions, instead of lithium in the lithiumsalts, an alkali metal (e.g., sodium or potassium) or an alkaline-earthmetal (e.g., calcium, strontium, barium, beryllium, or magnesium) may beused as the electrolyte.

Furthermore, an additive such as VC, propane sultone (PS),tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), or LiBOB may beadded to the electrolytic solution. The concentration of such anadditive in the whole solvent is, for example, higher than or equal to0.1 wt % and lower than or equal to 5 wt %.

With the use of the above solvent and the above electrolyte, anelectrolytic solution of the power storage device of one embodiment ofthe present invention can be formed.

<<Current Collector>>

There is no particular limitation on the current collector as long as ithas high conductivity without causing a significant chemical change in apower storage device. For example, the positive electrode currentcollector and the negative electrode current collector can each beformed using a metal such as stainless steel, gold, platinum, zinc,iron, nickel, copper, aluminum, titanium, tantalum, or manganese, analloy thereof, sintered carbon, or the like. Alternatively, copper orstainless steel that is coated with carbon, nickel, titanium, or thelike may be used. Alternatively, the current collectors can each beformed using an aluminum alloy to which an element that improves heatresistance, such as silicon, titanium, neodymium, scandium, ormolybdenum, is added. Still alternatively, a metal element that formssilicide by reacting with silicon can be used to form the currentcollectors. Examples of the metal element that forms silicide byreacting with silicon include zirconium, titanium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel.

An irreversible reaction with an electrolytic solution is sometimescaused on surfaces of the positive electrode current collector and thenegative electrode current collector. Thus, the positive electrodecurrent collector and the negative electrode current collectorpreferably have low reactivity to an electrolytic solution.

The positive electrode current collector and the negative electrodecurrent collector can each have any of various shapes including afoil-like shape, a plate-like shape (sheet-like shape), a net-likeshape, a cylindrical shape, a coil shape, a punching-metal shape, anexpanded-metal shape, a porous shape, and a shape of non-woven fabric asappropriate. The positive electrode current collector and the negativeelectrode current collector may each be formed to have microirregularities on the surface thereof in order to enhance adhesion tothe active material layer. The positive electrode current collector andthe negative electrode current collector each preferably have athickness of 5 μm to 30 μm inclusive.

An undercoat layer may be provided over part of a surface of the currentcollector. The undercoat layer is a coating layer provided to reducecontact resistance between the current collector and the active materiallayer or to improve adhesion between the current collector and theactive material layer. Note that the undercoat layer is not necessarilyformed over the entire surface of the current collector and may bepartly formed to have an island-like shape. In addition, the undercoatlayer may serve as an active material to have capacity. For theundercoat layer, a carbon material can be used, for example. Examples ofthe carbon material include graphite, carbon black such as acetyleneblack, and a carbon nanotube. Examples of the undercoat layer include ametal layer, a layer containing carbon and high molecular compounds, anda layer containing metal and high molecular compounds.

<<Active Material Layer>>

The active material layer includes the active material. An activematerial refers only to a material that is involved in insertion andextraction of ions that are carriers. In this specification and thelike, a layer including an active material is referred to as an activematerial layer. The active material layer may include a conductiveadditive and a binder in addition to the active material.

The positive electrode active material layer includes one or more kindsof positive electrode active materials. The negative electrode activematerial layer includes one or more kinds of negative electrode activematerials.

The positive electrode active material and the negative electrode activematerial have a central role in battery reactions of a power storagedevice, and receive and release carrier ions. To increase the lifetimeof the power storage device, the active materials preferably have alittle capacity involved in irreversible battery reactions, and havehigh charge and discharge efficiency.

For the positive electrode active material, a material into and fromwhich carrier ions such as lithium ions can be inserted and extractedcan be used. Examples of a positive electrode active material includematerials having an olivine crystal structure, a layered rock-saltcrystal structure, a spinel crystal structure, and a NASICON crystalstructure.

As the positive electrode active material, a compound such as LiCoO₂,LiNiO₂, or LiMn₂O₄, V₂O₅, Cr₂O₅, MnO₂, or LiFeO₂ can be used.

As an example of a material having an olivine crystal structure,lithium-containing complex phosphate (LiMPO₄ (general formula) (M is oneor more of Fe(II), Mn(II), Co(II), and Ni(II))) can be given. Typicalexamples of LiMPO₄ are compounds such as LiFePO₄, LiNiPO₄, LiCoPO₄,LiMnPO₄, LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄, LiFe_(a)Mn_(b)PO₄,LiNi_(a)Co_(b)PO₄, LiNi_(a)Mn_(b)PO₄ (a+b≤1, 0<a<1, and 0<b<1),LiFe_(c)Ni_(d)Co_(e)PO₄, LiFe_(c)Ni_(d)Mn_(e)PO₄,LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≤1, 0<c<1, 0<d<1, and 0<e<1), andLiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≤1, 0<f<1, 0<g<1, 0<h<1, and0<i<1).

For example, lithium iron phosphate (LiFePO₄) is preferable because itproperly has properties necessary for the positive electrode activematerial, such as safety, stability, high capacity density, highpotential, and the existence of lithium ions which can be extracted ininitial oxidation (charging).

Examples of a material with a layered rock-salt crystal structureinclude lithium cobalt oxide (LiCoO₂), LiNiO₂, LiMnO₂, Li₂MnO₃, aNiCo-containing material (general formula: LiNi_(x)Co_(1-x)O₂ (0<x<1))such as LiNi_(0.8)Co_(0.2)O₂, a NiMn-containing material (generalformula: LiNi_(x)Mn_(1-x)O₂ (0<x<1)) such as LiNi_(0.5)Mn_(0.5)O₂, aNiMnCo-containing material (also referred to as NMC) (general formula:LiNi_(x)Mn_(y)Co_(1-x-y)O₂ (x>0, y>0, x+y<1)) such asLiNi_(1/3)Mn_(1/3)Co_(1/3)O₂. Moreover,Li(Ni_(0.8)Co_(0.15)Al_(0.05))O₂, Li₂MnO₃—LiMO₂ (M=Co, Ni, or Mn), andthe like can be given as the examples.

In particular, LiCoO₂ is preferable because it has advantages such ashigh capacity, higher stability in the air than that of LiNiO₂, andhigher thermal stability than that of LiNiO₂.

Examples of a material with a spinel crystal structure include LiMn₂O₄,Li_(1+x)Mn_(2-x)O₄ (0<x<2), LiMn_(2-x)Al_(x)O₄ (0<x<2), andLiMn_(1.5)Ni_(0.5)O₄.

It is preferred that a small amount of lithium nickel oxide (LiNiO₂ orLiNi_(1-x)M_(x)O₂ (0<x<1, M=Co, Al, or the like)) be added to a materialwith a spinel crystal structure that contains manganese, such asLiMn₂O₄, in which case advantages such as inhibition of the dissolutionof manganese and the decomposition of an electrolytic solution can beobtained.

Alternatively, a lithium-containing complex silicate expressed byLi_((2-j))MSiO₄ (general formula) (M is one or more of Fe(II), Co(II),or Ni(II); 0≤j≤2) may be used as the positive electrode active material.Typical examples of the general formula Li_((2-j))MSiO₄ are compoundssuch as Li_((2-j))FeSiO₄, Li_((2-j))NiSiO₄, Li_((2-j))CoSiO₄,Li_((2-j))MnSiO₄, Li_((2-j))Fe_(k)Co_(l)SiO₄,Li_((2-j))Fe_(k)Mn_(l)SiO₄, Li_((2-j))Ni_(k)Co_(l)SiO₄,Li_((2-j))Ni_(k)Mn_(l)SiO₄ (k+1≤1, 0<k<1, and 0<1<1),Li_((2-j))Fe_(m)Ni_(n)Co_(q)SiO₄, Li_((2-j))Fe_(m)Ni_(n)Mn_(q)SiO₄,Li_((2-j))Ni_(m)Co_(n)Mn_(q)SiO₄ (m+n+q≤1, 0<m<1, 0<n<1, and 0<q<1), andLi_((2-j))Fe_(r)Ni_(s)Co_(t)Mn_(u)SiO₄ (r+s+t+u≤1, 0<r<1, 0<s<1, 0<t<1,and 0<u<1).

Still alternatively, a NASICON compound expressed by A_(x)M₂(XO₄)₃(general formula) (A=Li, Na, or Mg, M=Fe, Mn, Ti, V, Nb, or Al, X=S, P,Mo, W, As, or Si) can be used for the positive electrode activematerial. Examples of the NASICON compound are Fe₂(MnO₄)₃, Fe₂(SO₄)₃,and Li₃Fe₂(PO₄)₃.

Further alternatively, for example, a compound expressed by Li₂MPO₄F,Li₂MP₂O₇, or Li₅MO₄ (general formula) (M=Fe or Mn), a perovskitefluoride such as FeF₃, a metal chalcogenide (a sulfide, a selenide, or atelluride) such as TiS₂ and MoS₂, a lithium-containing material with aninverse spinel structure such as LiMVO₄, a vanadium oxide (V₂O₅, V₆O₁₃,LiV₃O₈, LiVOPO₄, or the like), a manganese oxide, or an organic sulfurcompound can be used as the positive electrode active material.

Further alternatively, any of the aforementioned materials may becombined to be used as the positive electrode active material. Forexample, a solid solution obtained by combining two or more of the abovematerials can be used as the positive electrode active material. Forexample, a solid solution of LiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂ and Li₂MnO₃can be used as the positive electrode active material.

In the case where carrier ions are alkali metal ions other than lithiumions, or alkaline-earth metal ions, a compound containing carriers suchas an alkali metal (e.g., sodium and potassium) or an alkaline-earthmetal (e.g., calcium, strontium, barium, beryllium, and magnesium)instead of lithium of the lithium compound, the lithium-containingcomplex phosphate, or the lithium-containing complex silicate may beused as the positive electrode active material.

The average diameter of primary particles of the positive electrodeactive material is preferably, for example, greater than or equal to 5nm and less than or equal to 100 μm.

For example, lithium-containing complex phosphate having an olivinecrystal structure used for the positive electrode active material has aone-dimensional lithium diffusion path, so that lithium diffusion isslow. Thus, in the case of using lithium-containing complex phosphatehaving an olivine crystal structure, the average diameter of particlesof the positive electrode active material is, for example, preferablygreater than or equal to 5 nm and less than or equal to 1 μm so that thecharge and discharge rate is increased. The specific surface area of thepositive electrode active material is, for example, preferably greaterthan or equal to 10 m²/g and less than or equal to 50 m²/g.

An active material having an olivine crystal structure is much lesslikely to be changed in the crystal structure by charging anddischarging and has a more stable crystal structure than, for example,an active material having a layered rock-salt crystal structure. Thus, apositive electrode active material having an olivine crystal structureis stable against operation such as overcharging. The use of such apositive electrode active material allows fabrication of a highly safepower storage device.

As the negative electrode active material, for example, a carbon-basedmaterial, an alloy-based material, or the like can be used.

Examples of the carbon-based material include graphite, graphitizingcarbon (soft carbon), non-graphitizing carbon (hard carbon), a carbonnanotube, graphene, carbon black, and the like. Examples of the graphiteinclude artificial graphite such as meso-carbon microbeads (MCMB),coke-based artificial graphite, or pitch-based artificial graphite andnatural graphite such as spherical natural graphite. In addition,examples of the shape of the graphite include a flaky shape and aspherical shape.

Graphite has a low potential substantially equal to that of a lithiummetal when lithium ions are intercalated into the graphite (while alithium-graphite intercalation compound is formed). For this reason, alithium-ion secondary battery can have a high operating voltage.Graphite is preferred because of its advantages described above, such asrelatively high capacity per unit volume, small volume expansion, lowcost, and safety greater than that of a lithium metal.

Here, a graphite material will be described. Graphite is a layeredcompound in which a plurality of graphene layers are stacked parallel toeach other by van der Waals forces. A surface of the graphite materialincludes a plane parallel to the graphene layer (also referred to as abasal plane) and a plane where edges of the plurality of graphene layersare arranged (also referred to as an edge plane). In the basal plane,one surface of the outmost layer of the graphene layers composinggraphite is exposed. In the edge plane, the edges of the plurality ofgraphene layers are exposed. In charging and discharging of a secondarybattery, the edge plane of the graphite material serves as a main gatefor lithium intercalation and deintercalation to and from the graphitematerial.

In the case where graphite is used for the negative electrode activematerial, the contact between the exposed portion of the edge plane andthe electrolytic solution containing PC might cause a side reactionbetween graphite and PC in charging and discharging. In sphericalnatural graphite used for the negative electrode active materialincluded in the power storage device of one embodiment of the presentinvention, a layer having lower crystallinity than a graphite layer isformed in contact with the edge plane; thus, a side reaction betweengraphite and PC can be inhibited in some cases.

For example, in the case where carrier ions are lithium ions, a materialincluding at least one of Mg, Ca, Ga, Si, Al, Ge, Sn, Pb, As, Sb, Bi,Ag, Au, Zn, Cd, Hg, In, and the like can be used as the alloy-basedmaterial. Such elements have a higher capacity than carbon. Inparticular, silicon has a high theoretical capacity of 4200 mAh/g, andtherefore, the capacity of the power storage device can be increased.Examples of an alloy-based material (compound-based material) using suchelements include Mg₂Si, Mg₂Ge, Mg₂Sn, SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂,Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb, CeSb₃, LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, andSbSn.

Alternatively, for the negative electrode active material, an oxide suchas SiO, SnO, SnO₂, titanium dioxide (TiO₂), lithium titanium oxide(Li₄Ti₅O₁₂), lithium-graphite intercalation compound (Li_(x)C₆), niobiumpentoxide (Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide (MoO₂) canbe used. Here, SiO is a compound containing silicon and oxygen. When theatomic ratio of silicon to oxygen is represented by α:β, α preferablyhas an approximate value of β. Here, when α has an approximate value ofβ, an absolute value of the difference between α and β is preferablyless than or equal to 20% of a value of β, more preferably less than orequal to 10% of a value of β.

Still alternatively, for the negative electrode active material,Li_(3-x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm³).

When a nitride containing lithium and a transition metal is used,lithium ions are contained in the negative electrode active material andthus the negative electrode active material can be used in combinationwith a material for a positive electrode active material that does notcontain lithium ions, such as V₂O₅ or Cr₃O₈. In the case of using amaterial containing lithium ions as a positive electrode activematerial, the nitride containing lithium and a transition metal can beused for the negative electrode active material by extracting thelithium ions contained in the positive electrode active material inadvance.

Alternatively, a material that causes a conversion reaction can be usedfor the negative electrode active material; for example, a transitionmetal oxide that does not cause an alloy reaction with lithium, such ascobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may beused. Other examples of the material which causes a conversion reactioninclude oxides such as Fe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂O₃, sulfides suchas CoS_(0.89), NiS, and CuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄,phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides such as FeF₃ andBiF₃.

The average diameter of primary particles of the negative electrodeactive material is preferably, for example, greater than or equal to 5nm and less than or equal to 100 μm.

The positive electrode active material layer and the negative electrodeactive material layer may each include a conductive additive.

Examples of the conductive additive include a carbon material, a metalmaterial, and a conductive ceramic material. Alternatively, a fibermaterial may be used as the conductive additive. The content of theconductive additive in the active material layer is preferably greaterthan or equal to 1 wt % and less than or equal to 10 wt %, morepreferably greater than or equal to 1 wt % and less than or equal to 5wt %.

A network for electric conduction can be formed in the electrode by theconductive additive. The conductive additive also allows maintaining ofa path for electric conduction between the negative electrode activematerial particles. The addition of the conductive additive to theactive material layer increases the electric conductivity of the activematerial layer.

Examples of the conductive additive include natural graphite, artificialgraphite such as mesocarbon microbeads, and carbon fiber. Examples ofcarbon fiber include mesophase pitch-based carbon fiber, isotropicpitch-based carbon fiber, carbon nanofiber, and carbon nanotube. Carbonnanotube can be formed by, for example, a vapor deposition method. Otherexamples of the conductive additive include carbon materials such ascarbon black (e.g., acetylene black (AB)), graphite (black lead)particles, graphene, graphene oxide, and fullerene. Alternatively, metalpowder or metal fibers of copper, nickel, aluminum, silver, gold, or thelike, a conductive ceramic material, or the like can be used.

Flaky graphene has an excellent electrical characteristic of highconductivity and excellent physical properties of high flexibility andhigh mechanical strength. Thus, the use of graphene as the conductiveadditive can increase contact points and the contact area of the activematerials.

Graphene is capable of making low-resistance surface contact and hasextremely high conductivity even with a small thickness. Therefore, evena small amount of graphene can efficiently form a conductive path in anactive material layer.

In the case where an active material with a small average particlediameter (e.g., 1 μm or less) is used, the specific surface area of theactive material is large and thus more conductive paths for the activematerial particles are needed. In such a case, it is particularlypreferred that graphene with extremely high conductivity that canefficiently form a conductive path even in a small amount is used.

The positive electrode active material layer and the negative electrodeactive material layer may each include a binder.

In this specification, the binder has a function of binding or bondingthe active materials and/or a function of binding or bonding the activematerial layer and the current collector. The binder is sometimeschanged in state during fabrication of an electrode or a battery. Forexample, the binder can be at least one of a liquid, a solid, and a gel.The binder is sometimes changed from a monomer to a polymer duringfabrication of an electrode or a battery.

As the binder, for example, a water-soluble high molecular compound canbe used. As the water-soluble high molecular compound, a polysaccharideor the like can be used. As the polysaccharide, a cellulose derivativesuch as carboxymethyl cellulose (CMC), methyl cellulose, ethylcellulose, hydroxypropyl cellulose, diacetyl cellulose, or regeneratedcellulose, starch, or the like can be used.

As the binder, a rubber material such as styrene-butadiene rubber (SBR),styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber,butadiene rubber, fluororubber, or ethylene-propylene-diene copolymercan be used. Any of these rubber materials may be used in combinationwith the aforementioned water-soluble high molecular compound. Sincethese rubber materials have rubber elasticity and easily expand andcontract, it is possible to obtain a highly reliable electrode that isresistant to stress due to expansion and contraction of an activematerial by charging and discharging, bending of the electrode, or thelike. On the other hand, the rubber materials have a hydrophobic groupand thus are unlikely to be soluble in water in some cases. In such acase, particles are dispersed in an aqueous solution without beingdissolved in water, so that increasing the viscosity of a compositioncontaining a solvent used for the formation of the active material layer(also referred to as an electrode binder composition) up to theviscosity suitable for application might be difficult. A water-solublehigh molecular compound having excellent viscosity modifying properties,such as a polysaccharide, can moderately increase the viscosity of thesolution and can be uniformly dispersed together with a rubber material.Thus, a favorable electrode with high uniformity (e.g., an electrodewith uniform electrode thickness or electrode resistance) can beobtained.

Alternatively, as the binder, a material such as PVdF, polystyrene,poly(methyl acrylate), poly(methyl methacrylate) (PMMA), sodiumpolyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO),polypropylene oxide, polyimide, polyvinyl chloride,polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene,polyethylene terephthalate, nylon, polyacrylonitrile (PAN),ethylene-propylene-diene polymer, polyvinyl acetate, or nitrocellulosecan be used.

Two or more of the above materials may be used in combination for thebinder.

The content of the binder in the active material layer is preferablygreater than or equal to 1 wt % and less than or equal to 10 wt %, morepreferably greater than or equal to 2 wt % and less than or equal to 8wt %, and still more preferably greater than or equal to 3 wt % and lessthan or equal to 5 wt %.

<<Exterior Body>>

It is preferred that the surface of the exterior body 509 that is incontact with the electrolytic solution 508, i.e., the inner surface ofthe exterior body 509, does not react with the electrolytic solution 508significantly. When moisture enters the power storage device 500 fromthe outside, a reaction between a component of the electrolytic solution508 or the like and water might occur. Thus, the exterior body 509preferably has low moisture permeability.

As the exterior body 509, a film having a three-layer structure can beused, for example. In the three-layer structure, a highly flexible metalthin film of aluminum, stainless steel, copper, nickel, or the like isprovided over a film formed using polyethylene, polypropylene,polycarbonate, ionomer, polyamide, or the like, and an insulatingsynthetic resin film of a polyamide-based resin, a polyester-basedresin, or the like is provided as the outer surface of the exterior bodyover the metal thin film can be used. With such a three-layer structure,the passage of an electrolytic solution and a gas can be blocked and aninsulating property and resistance to the electrolytic solution can beprovided. The exterior body is folded inside in two, or two exteriorbodies are stacked with the inner surfaces facing each other, in whichcase application of heat melts the materials on the overlapping innersurfaces to cause fusion bonding between the two exterior bodies. Inthis manner, a sealing structure can be formed.

A portion where the sealing structure is formed by fusion bonding or thelike of the exterior body is referred to as a sealing portion. In thecase where the exterior body is folded inside in two, the sealingportion is formed in the place other than the fold, and a first regionof the exterior body and a second region of the exterior body thatoverlaps with the first region are fusion-bonded, for example. In thecase where two exterior bodies are stacked, the sealing portion isformed along the entire outer region by heat fusion bonding or the like.

The power storage device 500 can be flexible by using the exterior body509 with flexibility. When the power storage device 500 has flexibility,it can be used in an electronic device at least part of which isflexible, and the power storage device 500 can be bent as the electronicdevice changes its form.

Note that in one embodiment of the present invention, a graphenecompound can be used in a component of the power storage device. Asdescribed later, when modification is performed, the structure andcharacteristics of a graphene compound can be selected from a widerrange of alternatives. Thus, a preferable property can be exhibited inaccordance with a component in which a graphene compound is to be used.Moreover, a graphene compound has high mechanical strength and thereforecan be used in a component of a flexible power storage device. Graphenecompounds will be described below.

Graphene has carbon atoms arranged in one atomic layer. A π bond existsbetween the carbon atoms. Graphene including two or more and one hundredor less layers is referred to as multilayer graphene in some cases. Thelength in the longitudinal direction or the length of the major axis ina plane in each of graphene and multilayer graphene is greater than orequal to 50 nm and less than or equal to 100 μm or greater than or equalto 800 nm and less than or equal to 50 μm.

In this specification and the like, a compound including graphene ormultilayer graphene as a basic skeleton is referred to as a graphenecompound. Graphene compounds include graphene and multilayer graphene.

Graphene compounds will be detailed below.

A graphene compound is, for example, a compound where graphene ormultilayer graphene is modified with an atom other than carbon or anatomic group with an atom other than carbon. A graphene compound may bea compound where graphene or multilayer graphene is modified with anatomic group composed mainly of carbon, such as an alkyl group oralkylene. An atomic group that modifies graphene or multilayer grapheneis referred to as a substituent, a functional group, a characteristicgroup, or the like in some cases. Modification in this specification andthe like refers to introduction of an atom other than carbon, an atomicgroup with an atom other than carbon, or an atomic group composed mainlyof carbon to graphene, multilayer graphene, a graphene compound, orgraphene oxide (described later) by a substitution reaction, an additionreaction, or other reactions.

Note that the surface and the rear surface of graphene may be modifiedwith different atoms or atomic groups. In multilayer graphene, multiplelayers may be modified with different atoms or atomic groups.

An example of the above-described graphene modified with an atom or anatomic group is graphene or multilayer graphene that is modified withoxygen or a functional group containing oxygen. Examples of a functionalgroup containing oxygen include an epoxy group, a carbonyl group such asa carboxyl group, and a hydroxyl group. A graphene compound modifiedwith oxygen or a functional group containing oxygen is referred to asgraphene oxide in some cases. In this specification, graphene oxidesinclude multilayer graphene oxides.

As an example of modification of graphene oxide, silylation of grapheneoxide will be described. First, in a nitrogen atmosphere, graphene oxideis put in a container, n-butylamine (C₄H₉NH₂) is added to the container,and stirring is performed for one hour with the temperature kept at 60°C. Then, toluene is added to the container, alkyltrichlorosilane isadded thereto as a silylating agent, and stirring is performed in anitrogen atmosphere for five hours with the temperature kept at 60° C.Then, toluene is further added to the container, and the resultingsolution is suction-filtrated to give solid powder. The powder isdispersed in ethanol, and the resulting solution is suction-filtered togive solid powder. The powder is dispersed in acetone, and the resultingsolution is suction-filtered to give solid powder. A liquid of the solidpowder is vaporized to give silylated graphene oxide.

The modification is not limited to silylation, and silylation is notlimited to the above-described method. Furthermore, the modification isnot limited to introduction of an atom or an atomic group of one kind,and the modification of two or more types may be performed to introduceatoms or atomic groups of two or more kinds. By introducing a givenatomic group to a graphene compound, the physical property of thegraphene compound can be changed. Therefore, by performing desirablemodification in accordance with the use of a graphene compound, adesired property of the graphene compound can be exhibitedintentionally.

A formation method example of graphene oxide will be described below.Graphene oxide can be obtained by oxidizing the aforementioned grapheneor multilayer graphene. Alternatively, graphene oxide can be obtained bybeing separated from graphite oxide. Graphite oxide can be obtained byoxidizing graphite. The graphene oxide may further be modified with theabove-mentioned atom or atomic group.

A compound that can be obtained by reducing graphene oxide is referredto as reduced graphene oxide (RGO) in some cases. In RGO, in some cases,all oxygen atoms contained in the graphene oxide are not extracted andpart of them remains in a state of oxygen or an atomic group containingoxygen that is bonded to carbon. In some cases, RGO includes afunctional group, e.g., an epoxy group, a carbonyl group such as acarboxyl group, or a hydroxyl group.

A graphene compound may have a sheet-like shape where a plurality ofgraphene compounds partly overlap with each other. Such a graphenecompound is referred to as a graphene compound sheet in some cases. Thegraphene compound sheet has, for example, an area with a thicknesslarger than or equal to 0.33 nm and smaller than or equal to 10 mm,preferably larger than or equal to 0.34 nm and smaller than or equal to10 μm. The graphene compound sheet may be modified with an atom otherthan carbon, an atomic group containing an atom other than carbon, anatomic group composed mainly of carbon such as an alkyl group, or thelike. A plurality of layers in the graphene compound sheet may bemodified with different atoms or atomic groups.

A graphene compound may have a five-membered ring composed of carbonatoms or a poly-membered ring that is a seven- or more-membered ringcomposed of carbon atoms, in addition to a six-membered ring composed ofcarbon atoms. In the neighborhood of a poly-membered ring which is aseven- or more-membered ring, a region through which a lithium ion canpass may be generated.

Furthermore, a plurality of graphene compounds may be gathered to form asheet-like shape, for example.

A graphene compound has a planar shape, thereby enabling surfacecontact.

In some cases, a graphene compound has high conductivity even when it isthin. The contact area between graphene compounds or between a graphenecompound and an active material can be increased by surface contact.Thus, even with a small amount of a graphene compound per volume, aconductive path can be formed efficiently.

In contrast, a graphene compound may also be used as an insulator. Forexample, a graphene compound sheet can be used as a sheet-likeinsulator. Graphene oxide, for example, has a more excellent insulationproperty than a graphene compound that is not oxidized, in some cases. Agraphene compound modified with an atomic group may have an improvedinsulation property, depending on the type of the modifying atomicgroup.

A graphene compound in this specification and the like may include aprecursor of graphene. The precursor of graphene refers to a substanceused to form graphene. The precursor of graphene may contain theabove-described graphene oxide, graphite oxide, or the like.

Graphene containing an alkali metal or an element other than carbon,such as oxygen, is referred to as a graphene analog in some cases. Inthis specification and the like, graphene compounds include grapheneanalogs.

A graphene compound in this specification and the like may include anatom, an atomic group, and ions of them between the layers. The physicalproperties, such as electric conductivity and ionic conductivity, of agraphene compound sometimes change when an atom, an atomic group, andions of them exist between layers of the compound. In addition, adistance between the layers is increased in some cases.

A graphene compound has excellent electrical characteristics of highconductivity and excellent physical properties of high flexibility andhigh mechanical strength in some cases. A modified graphene compound canhave extremely low conductivity and serve as an insulator depending onthe type of the modification. A graphene compound has a planar shape. AGraphene compound enables low-resistance surface contact.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 2

In this embodiment, electronic devices of embodiments of the presentinvention will be described with reference to FIG. 12A to 12C to FIGS.15A to 15C.

Structural Example 1 of Smartwatch

FIG. 12A is a perspective view of a watch-type portable informationterminal (also called a smartwatch) 700. The portable informationterminal 700 includes a housing 701, a display panel 702, a clasp 703,bands 705A and 705B, and operation buttons 711 and 712.

The display panel 702 mounted in the housing 701 doubling as a bezelincludes a rectangular display region. The display region has a curvedsurface. The display panel 702 preferably has flexibility. Note that thedisplay region may be non-rectangular.

The bands 705A and 705B are connected to the housing 701. The clasp 703is connected to the band 705A. The band 705A and the housing 701 areconnected to each other with a pin such that they can pivot around thepin at a connection portion, for example. In a similar manner, the band705B and the housing 701 are connected to each other and the band 705Aand the clasp 703 are connected to each other.

FIGS. 12B and 12C are perspective views of the band 705A and a powerstorage device 750, respectively. The band 705A includes the powerstorage device 750. As the power storage device 750, the power storagedevice 500 described in Embodiment 1 can be used, for example. The powerstorage device 750 is embedded in the band 705A, and part of thepositive electrode lead 751 and part of the negative electrode lead 752protrude from the band 705A (see FIG. 12B). The positive electrode lead751 and the negative electrode lead 752 are electrically connected tothe display panel 702. The surface of the power storage device 750 iscovered with an exterior body 753 (see FIG. 12C). Note that the pin mayfunction as an electrode. Specifically, the positive electrode lead 751and the display panel 702 may be electrically connected to each otherthrough the pin that connects the band 705A and the housing 701, and thenegative electrode lead 752 and the display panel 702 may beelectrically connected to each other through the pin. In that case, thestructure of the connection portion between the band 705A and thehousing 701 can be simplified.

The power storage device 750 has flexibility. Specifically, a surface ofthe exterior body 753 preferably has projections and depressions formedby the embossing described in Embodiment 1. Furthermore, the powerstorage device 750 preferably has sliding planes of the power storagedevice 500 illustrated in FIGS. 4A and 4B.

The band 705A can be formed so as to incorporate the power storagedevice 750. For example, the power storage device 750 is set in a moldthat the outside shape of the band 705A fits and a material of the band705A is poured in the mold and cured, so that the band 705A illustratedin FIG. 12B can be formed.

In the case where a rubber material is used as the material of the band705A, rubber is cured through heat treatment. For example, in the casewhere fluorine rubber is used as a rubber material, it is cured throughheat treatment at 170° C. for 10 minutes. In the case where siliconerubber is used as a rubber material, it is cured through heat treatmentat 150° C. for 10 minutes. The power storage device of one embodiment ofthe present invention has high heat resistance, which can inhibitbreakage and degradation of the charge and discharge characteristics dueto heat treatment performed when the power storage device and the rubbermaterial are integrally formed.

Examples of the material of the band 705A include fluorine rubber,silicone rubber, fluorosilicone rubber, and urethane rubber.

Note that energization of the power storage device 750, including aging,is preferably performed after the power storage device 750 is formed tobe incorporated in the band 705A. In other words, heat treatment ispreferably performed on the power storage device 500 described inEmbodiment 1 before energization of the power storage device 500. Theheat treatment is preferably performed at 110° C. to 190° C. inclusivefor a period of time suitable for vulcanization of the rubber material,for example, at 170° C. for 10 minutes. This can inhibit degradation ofthe charge and discharge characteristics of the power storage device 500due to heat treatment.

The portable information terminal 700 in FIG. 12A can have a variety offunctions such as a function of displaying a variety of data (e.g., astill image, a moving image, and a text image) on the display region, atouch panel function, a function of displaying a calendar, date, time,and the like, a function of controlling processing with a variety ofsoftware (programs), a wireless communication function, a function ofbeing connected to a variety of computer networks with a wirelesscommunication function, a function of transmitting and receiving avariety of data with a wireless communication function, and a functionof reading out a program or data stored in a recording medium anddisplaying it on the display region.

The housing 701 can include a speaker, a sensor (a sensor having afunction of measuring force, displacement, position, speed,acceleration, angular velocity, rotational frequency, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, odor, or infrared rays), amicrophone, and the like. Note that the portable information terminal700 can be manufactured using the light-emitting element for the displaypanel 702.

Although FIGS. 12A to 12C illustrate the example where the power storagedevice 750 is incorporated in the band 705A, the power storage device750 may be incorporated in the band 705B. The band 705B can be formedusing a material similar to that of the band 705A.

The rubber material used for the band 705A preferably has high chemicalresistance. Specifically, the rubber material preferably has lowreactivity to an electrolytic solution contained in the power storagedevice 750.

When the band 705A is cracked or chipped despite of its high chemicalresistance, a user of the portable information terminal 700 might touchthe electrolytic solution that leaks from the power storage device 750.In the case where the portable information terminal 700 has a functionof detecting leakage of the electrolytic solution, the user can stopoperation of the portable information terminal 700 and remove it as soonas the electrolytic solution leakage is detected. Consequently, theportable information terminal 700 can be highly safe.

Structural Example 2 of Smartwatch

FIG. 13A is a perspective view of a band 735A having a structuredifferent form that of the band 705A illustrated in FIG. 12B. A housing731 connected to the band 735A includes a leakage detection circuit (notillustrated) having a function of detecting leakage of the electrolyticsolution of the power storage device (see FIG. 12A). Note that aperspective view of a portable information terminal 730 including theleakage detection circuit is similar to that of the portable informationterminal 700.

The band 735A includes a power storage device 760. The power storagedevice 760 is embedded in the band 735A, and part of the positiveelectrode lead 751, part of the negative electrode lead 752, part of aterminal 761, and part of a terminal 762 protrude from the band 735A.The positive electrode lead 751 and the negative electrode lead 752 areelectrically connected to the display panel 702. The terminal 761 andthe terminal 762 are electrically connected to the above leakagedetection circuit, for example.

FIG. 13B is a perspective view of the power storage device 760. FIG. 13Bis an enlarged view of FIG. 13A for clarification. The power storagedevice 760 is different from the power storage device 750 in FIG. 12C inincluding the terminal 761, terminal 762, a wiring 771, and a wiring772. The terminal 761 is electrically connected to the wiring 771. Theterminal 762 is electrically connected to the wiring 772.

Although the wiring 771 and the wiring 772 are indicated by differenthatch patterns for clarification in FIG. 13B, the wiring 771 and thewiring 772 are preferably formed using the same material to achieve costreduction. Although the terminal 761 and the wiring 771 are indicated bythe same hatching pattern and the terminal 762 and the wiring 772 areindicated by the same hatching pattern, the terminal 761 and the wiring771 may be formed using different materials and the terminal 762 and thewiring 772 may be formed using different materials.

The wiring 771 and the wiring 772 are provided on a surface of theexterior body 753 with a predetermined gap therebetween (see FIG. 13B).If the electrolytic solution leaks to a surface of the exterior body753, the wiring 771 and the wiring 772 are electrically connected toeach other through the electrolytic solution, whereby the leakagedetection circuit can detect leakage of the electrolytic solution.

Although the wiring 771 and the wiring 772 each having a linear shapeare provided in the direction of the long axis of the power storagedevice 760 in FIG. 13B, one embodiment of the present invention is notlimited thereto. For example, as illustrated in FIG. 13C, the wiring 771and the wiring 772 each having a comb-like shape may be provided with agap therebetween so as to engage with each other.

Although FIG. 13C illustrates the example where the wirings 771 and 772are provided only on the top surface of the exterior body 753, thewirings 771 and 772 are preferably provided on the entire surface of theexterior body 753 as illustrated in FIG. 14A. FIG. 14B is a perspectiveview of the back surface of the power storage device 760 illustrated inFIG. 14A.

The wirings 771 and 772 each preferably have a small thickness and asmall width, in which case the flexibility of the power storage device760 can be ensured. For example, the power storage device 760 preferablyincludes a region in which the thickness of each of the wirings 771 and772 is 5 μm to 500 μm inclusive. Furthermore, it is preferred that thegap between the wiring 771 and the wiring 772 be small and the width ofeach of the wiring 771 and the wiring 772 be small, in which case even asmall amount of electrolytic solution leakage can be detected. Forexample, the power storage device 760 preferably includes a region inwhich the length of the gap between the wirings 771 and 772 is 0.5 mmand 20 mm inclusive. Furthermore, the power storage device 760preferably includes a region in which the width of each of the wiring771 and the wiring 772 is 0.5 mm and 5 mm inclusive. When the occupationarea of the wirings 771 and 772 in the surface of the exterior body 753is excessively small, the detection of electrolytic solution leakage forthe entire surface of the exterior body 753 is not possible in somecases, whereas when the occupation area is excessively large, theflexibility of the power storage device 760 is low in some cases. In thepower storage device 760, the proportion of the surface area of thewirings 771 and 772 except side surfaces thereof (the surfaces incontact with the exterior body 753) to the surface area of the exteriorbody 753 is preferably 5% to 50% inclusive.

The wirings 771 and 772 preferably include a material having highductility or high malleability. In particular, the use of a materialhaving both high ductility and high malleability can suppress breakageof the wirings 771 and 772 due to curving of the power storage device760. Examples of the material having both high ductility and highmalleability include a metal material such as gold, silver, platinum,iron, nickel, copper, aluminum, zinc, and tin and an alloy containingany of the metal materials.

<<Leakage Detecting Method>>

An example of a method for detecting electrolytic solution leakage inthe portable information terminal 730 will be described below. FIG. 15Ais a block diagram of the configuration of the portable informationterminal 730 when the electrolytic solution 736 leaks. In FIG. 15A,lines with arrows indicate the directions in which a wired signal or awireless signal is transmitted. Thus, components connected by thecorresponding line are electrically connected to each other in somecases. Lines without an arrow indicate wirings, and components connectedby the corresponding line are electrically connected to each other.

The portable information terminal 730 includes a leakage detectioncircuit 732, a power source 733, an ammeter 734, the wiring 771, and thewiring 772 (see FIG. 15A). The leakage detection circuit 732, the powersource 733, and the ammeter 734 are included in the housing 731. Thepower source 733 and the ammeter 734 may be included in the leakagedetection circuit 732. The portable information terminal 730 alsoincludes a functional circuit 739. The functional circuit 739 includesthe speaker, the sensor, the microphone, and the like. The functionalcircuit 739 is included in the housing 731.

The wirings 771 and 772 are electrically connected to the power source733, and a given voltage is applied between the wiring 771 and thewiring 772 (see FIG. 15A). The on/off of the power source 733 iscontrolled by the leakage detection circuit 732.

FIG. 15B is a flow chart showing the flow of detection of electrolyticsolution leakage in the portable information terminal 730. The methodfor detecting electrolytic solution leakage in the portable informationterminal 730 includes the following four steps, for example.

When the electrolytic solution 736 of the power storage device 760leaks, the electrolytic solution 736 is attached to a surface of theexterior body 753 (see FIG. 15A and S1 in FIG. 15B). The electrolyticsolution 736 attached to the surface of the exterior body 753 comes incontact with the wiring 771 and the wiring 772, whereby a current flowsthrough the wiring 771 and the wiring 772 (see S2 in FIG. 15B). Ondetecting the current, the ammeter 734 connected in parallel to thewiring 772 outputs a detection signal to the leakage detection circuit732 (see S3 in FIG. 15B). The leakage detection circuit 732 terminatesthe operation of the display panel 702 and/or the functional circuit 739in response to the detection signal (see S4 in FIG. 15B).

Although FIG. 15A illustrates the example where the ammeter 734 isconnected to the wiring 772, the ammeter 734 may be connected to thewiring 771. Furthermore, the power source 733 and the ammeter 734 may beincluded in the leakage detection circuit 732, and the leakage detectioncircuit 732 may be electrically connected to the wirings 771 and 772(see FIG. 15C). In that case, the leakage detection circuit 732 has afunction of applying a predetermined voltage to the wirings 771 and 772and a function of detecting a current flowing through the wirings 771and 772.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 3

In this embodiment, flexible power storage devices that are embodimentsof the present invention will be described with reference to FIGS. 16Aand 16B to FIG. 23. The power storage device of one embodiment of thepresent invention may have a curved shape. The power storage device ofone embodiment of the present invention may be flexible and capable ofbeing used while being curved and while being not curved.

<Structural Example 1>

FIG. 16A is a perspective view of a secondary battery 200 and FIG. 16Bis a top view of the secondary battery 200.

FIG. 17A is a cross-sectional view along dashed-dotted line C1-C2 inFIG. 16B, and FIG. 17B is a cross-sectional view along dashed-dottedline C3-C4 in FIG. 16B. Note that FIGS. 17A and 17B do not illustrateall components for clarity of the drawings.

The secondary battery 200 includes a positive electrode 211, a negativeelectrode 215, and a separator 203. The secondary battery 200 furtherincludes a positive electrode lead 221, a negative electrode lead 225,and an exterior body 207.

The positive electrode 211 and the negative electrode 215 each include acurrent collector and an active material layer. The positive electrode211 and the negative electrode 215 are provided such that the activematerial layers face each other with the separator 203 providedtherebetween.

One of the electrodes (the positive electrode 211 and the negativeelectrode 215) of the secondary battery 200 that is positioned on theouter diameter side of a curved portion is preferably longer than theother electrode that is positioned on the inner diameter side of thecurved portion, in the direction in which the electrode is curved. Withsuch a structure, ends of the positive electrode 211 and those of thenegative electrode 215 are aligned when the secondary battery 200 iscurved with a certain curvature. That is, the entire region of thepositive electrode active material layer included in the positiveelectrode 211 can face the negative electrode active material layerincluded in the negative electrode 215. Thus, positive electrode activematerials included in the positive electrode 211 can efficientlycontribute to a battery reaction. Therefore, the capacity of thesecondary battery 200 per volume can be increased. Such a structure isparticularly effective in a case where the curvature of the secondarybattery 200 is fixed in using the secondary battery 200.

The positive electrode lead 221 is electrically connected to a pluralityof positive electrodes 211. The negative electrode lead 225 iselectrically connected to a plurality of negative electrodes 215. Thepositive electrode lead 221 and the negative electrode lead 225 eachinclude a sealing layer 220.

The exterior body 207 covers a plurality of positive electrodes 211, aplurality of negative electrodes 215, and a plurality of separators 203.The secondary battery 200 includes an electrolytic solution (not shown)in a region covered with the exterior body 207. Three sides of theexterior body 207 are bonded, whereby the secondary battery 200 issealed.

In FIGS. 17A and 17B, the separators 203 each having a strip-like shapeare used and each pair of the positive electrode 211 and the negativeelectrode 215 sandwich the separator 203; however, one embodiment of thepresent invention is not limited to this structure. One separator sheetmay be folded in zigzag (or into a bellows shape) or wound so that theseparator is positioned between the positive electrode and the negativeelectrode.

An example of a method for fabricating the secondary battery 200 isillustrated in FIGS. 19A to 19D. FIG. 18 is a cross-sectional view alongdashed-dotted line C1-C2 in FIG. 16B of the case of employing thismanufacturing method.

First, the negative electrode 215 is positioned over the separator 203(FIG. 19A) such that the negative electrode active material layer of thenegative electrode 215 overlaps with the separator 203.

Then, the separator 203 is folded to overlap with the negative electrode215. Next, the positive electrode 211 overlaps with the separator 203(FIG. 19B) such that the positive electrode active material layer of thepositive electrode 211 overlaps with the separator 203 and the negativeelectrode active material layer. Note that in the case of using anelectrode in which one surface of a current collector is provided withan active material layer, the positive electrode active material layerof the positive electrode 211 and the negative electrode active materiallayer of the negative electrode 215 are positioned to face each otherwith the separator 203 provided therebetween.

In the case where the separator 203 is formed using a material that canbe thermally welded, such as polypropylene, a region where the separator203 overlaps with itself is thermally welded and then another electrodeoverlaps with the separator 203, whereby the slippage of the electrodein the fabrication process can be suppressed. Specifically, a regionwhich does not overlap with the negative electrode 215 or the positiveelectrode 211 and in which the separator 203 overlaps with itself, e.g.,a region denoted as 203 a in FIG. 19B, is preferably thermally welded.

By repeating the above steps, the positive electrode 211 and thenegative electrode 215 can overlap with each other with the separator203 provided therebetween as illustrated in FIG. 19C.

Note that a plurality of positive electrodes 211 and a plurality ofnegative electrodes 215 may be placed to be alternately sandwiched bythe separator 203 that is repeatedly folded in advance.

Then, as illustrated in FIG. 19C, a plurality of positive electrodes 211and a plurality of negative electrodes 215 are covered with theseparator 203.

Furthermore, the region where the separator 203 overlaps with itself,e.g., a region 203 b in FIG. 19D, is thermally welded as illustrated inFIG. 19D, whereby a plurality of positive electrodes 211 and a pluralityof negative electrodes 215 are covered with and tied with the separator203.

Note that a plurality of positive electrodes 211, a plurality ofnegative electrodes 215, and the separator 203 may be tied with abinding material.

Since the positive electrodes 211 and the negative electrodes 215 arestacked in the above process, one separator 203 has a region sandwichedbetween a plurality of positive electrodes 211 and a plurality ofnegative electrodes 215 and a region covering a plurality of positiveelectrodes 211 and a plurality of negative electrodes 215.

In other words, the separator 203 included in the secondary battery 200in FIG. 18 and FIG. 19D is a single separator which is partly folded. Inthe folded regions of the separator 203, a plurality of positiveelectrodes 211 and a plurality of negative electrodes 215 are provided.

<Structural Example 2>

FIG. 20A is a perspective view of a secondary battery 250 and FIG. 20Bis a top view of the secondary battery 250. Furthermore, FIG. 20C1 is across-sectional view of a first electrode assembly 230 and FIG. 20C2 isa cross-sectional view of a second electrode assembly 231.

The secondary battery 250 includes the first electrode assembly 230, thesecond electrode assembly 231, and the separator 203. The secondarybattery 250 further includes the positive electrode lead 221, thenegative electrode lead 225, and the exterior body 207.

As illustrated in FIG. 20C1, in the first electrode assembly 230, apositive electrode 211 a, the separator 203, a negative electrode 215 a,the separator 203, and the positive electrode 211 a are stacked in thisorder. The positive electrode 211 a and the negative electrode 215 aeach include active material layers on opposite surfaces of a currentcollector.

As illustrated in FIG. 20C2, in the second electrode assembly 231, anegative electrode 215 a, the separator 203, the positive electrode 211a, the separator 203, and the negative electrode 215 a are stacked inthis order. The positive electrode 211 a and the negative electrode 215a each include active material layers on opposite surfaces of a currentcollector.

In other words, in each of the first electrode assembly 230 and thesecond electrode assembly 231, the positive electrode and the negativeelectrode are provided such that the active material layers face eachother with the separator 203 provided therebetween.

The positive electrode lead 221 is electrically connected to a pluralityof positive electrodes 211. The negative electrode lead 225 iselectrically connected to a plurality of negative electrodes 215. Thepositive electrode lead 221 and the negative electrode lead 225 eachinclude the sealing layer 220.

FIG. 21 is an example of a cross-sectional view along dashed-dotted lineD1-D2 in FIG. 20B. Note that FIG. 21 does not illustrate all componentsfor clarity of the drawings.

As illustrated in FIG. 21, the secondary battery 250 has a structure inwhich a plurality of first electrode assemblies 230 and a plurality ofsecond electrode assemblies 231 are covered with the wound separator203.

The exterior body 207 covers a plurality of first electrode assemblies230, a plurality of second electrode assemblies 231, and the separator203. The secondary battery 200 includes an electrolytic solution (notshown) in a region covered with the exterior body 207. Three sides ofthe exterior body 207 are bonded, whereby the secondary battery 200 issealed.

An example of a method for fabricating the secondary battery 250 isillustrated in FIGS. 22A to 22D.

First, the first electrode assembly 230 is positioned over the separator203 (FIG. 22A).

Then, the separator 203 is folded to overlap with the first electrodeassembly 230. After that, two second electrode assemblies 231 arepositioned over and under the first electrode assembly 230 with theseparator 203 positioned between each of the second electrode assemblies231 and the first electrode assembly 230 (FIG. 22B).

Then, the separator 203 is wound to cover the two second electrodeassemblies 231. Moreover, two first electrode assemblies 230 arepositioned over and under the two second electrode assemblies 231 withthe separator 203 positioned between each of the first electrodeassemblies 230 and each of the second electrode assemblies 231 (FIG.22C).

Then, the separator 203 is wound to cover the two first electrodeassemblies 230 (FIG. 22D).

Since a plurality of first electrode assemblies 230 and a plurality ofsecond electrode assemblies 231 are stacked in the above process, theseelectrode assemblies are each positioned surrounded with the spirallywound separator 203.

Note that the outermost electrode preferably does not include an activematerial layer on the outer side.

Although FIGS. 20C1 and 20C2 each illustrate a structure in which theelectrode assembly includes three electrodes and two separators, oneembodiment of the present invention is not limited to this structure.The electrode assembly may include four or more electrodes and three ormore separators. A larger number of electrodes lead to higher capacityof the secondary battery 250. Alternatively, the electrode assembly mayinclude two electrodes and one separator. A smaller number of electrodesenable higher resistance of the secondary battery against bending.Although FIG. 21 illustrates the structure in which the secondarybattery 250 includes three first electrode assemblies 230 and two secondelectrode assemblies 231, one embodiment of the present invention is notlimited to this structure. The number of the electrode assemblies may beincreased. A larger number of electrode assemblies lead to highercapacity of the secondary battery 250. The number of the electrodeassemblies may be decreased. A smaller number of electrode assembliesenable higher resistance of the secondary battery against bending.

FIG. 23 illustrates another example of a cross-sectional view alongdashed-dotted line D1-D2 in FIG. 20B. As illustrated in FIG. 23, theseparator 203 may be folded into a bellows shape so that the separator203 is positioned between the first electrode assembly 230 and thesecond electrode assembly 231.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 4

In this embodiment, application examples of the power storage device ofone embodiment of the present invention will be described with referenceto FIGS. 24A to 24F to FIGS. 28A and 28B.

The power storage device of one embodiment of the present invention canbe used for an electronic device or a lighting device, for example. Thepower storage device of one embodiment of the present invention hasexcellent charge and discharge characteristics. Therefore, theelectronic device or the lighting device can be used for a long time bya single charge. Moreover, since a decrease in capacity with anincreasing number of charge and discharge cycles is inhibited, the timebetween charges is unlikely to be reduced by repetitive charge.Furthermore, the power storage device of one embodiment of the presentinvention exhibits excellent charge and discharge characteristics andhigh long-term reliability and is highly safe at a wide range oftemperature including high temperatures, so that the safety andreliability of an electronic device or a lighting device can beimproved.

Examples of electronic devices include a television set (also referredto as a television or a television receiver), a monitor of a computer orthe like, a digital camera, a digital video camera, a digital photoframe, a mobile phone (also referred to as a mobile phone device), aportable game machine, a portable information terminal, an audioreproducing device, a large game machine such as a pinball machine, andthe like.

Since the power storage device of one embodiment of the presentinvention is flexible, the power storage device or an electronic deviceor a lighting device using the power storage device can be incorporatedalong a curved inside/outside wall surface of a house or a building or acurved interior/exterior surface of a motor vehicle.

FIG. 24A illustrates an example of a mobile phone. A mobile phone 7400is provided with a display portion 7402 incorporated in a housing 7401,an operation button 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400includes a power storage device 7407.

FIG. 24B illustrates the mobile phone 7400 in the state of being bent.When the whole mobile phone 7400 is bent by the external force, thepower storage device 7407 included in the mobile phone 7400 is alsobent. The power storage device 7407 is a thin power storage device. Thepower storage device 7407 is fixed in a state of being bent. FIG. 24Cillustrates the power storage device 7407 in the state of being bent

FIG. 24D illustrates an example of a bangle display device. A portabledisplay device 7100 includes a housing 7101, a display portion 7102, anoperation button 7103, and a power storage device 7104. FIG. 24Eillustrates the bent power storage device 7104.

FIG. 24F illustrates an example of a watch-type portable informationterminal. A portable information terminal 7200 includes a housing 7201,a display portion 7202, a band 7203, a buckle 7204, an operation button7205, an input output terminal 7206, and the like.

The portable information terminal 7200 is capable of executing a varietyof applications such as mobile phone calls, e-mailing, viewing andediting text, music reproduction, Internet communication, and a computergame.

The display surface of the display portion 7202 is curved, and imagescan be displayed on the curved display surface. In addition, the displayportion 7202 includes a touch sensor, and operation can be performed bytouching the screen with a finger, a stylus, or the like. For example,by touching an icon 7207 displayed on the display portion 7202,application can be started.

With the operation button 7205, a variety of functions such as timesetting, power ON/OFF, ON/OFF of wireless communication, setting andcancellation of silent mode, and setting and cancellation of powersaving mode can be performed. For example, the functions of theoperation button 7205 can be set freely by the operating systemincorporated in the portable information terminal 7200.

Furthermore, the portable information terminal 7200 can employ nearfield communication, which is a communication method based on anexisting communication standard. In that case, for example, mutualcommunication between the portable information terminal 7200 and aheadset capable of wireless communication can be performed, and thushands-free calling is possible.

Moreover, the portable information terminal 7200 includes the inputoutput terminal 7206, and data can be directly transmitted to andreceived from another information terminal via a connector. In addition,charging via the input output terminal 7206 is possible. Note that thecharging operation may be performed by wireless power feeding withoutusing the input output terminal 7206.

The display portion 7202 of the portable information terminal 7200 isprovided with the power storage device of one embodiment of the presentinvention. For example, the power storage device 7104 illustrated inFIG. 24E that is in the state of being curved can be provided in thehousing 7201. Alternatively, the power storage device 7104 illustratedin FIG. 24E can be provided in the band 7203 such that it can be curved.

FIG. 25A illustrates an example of a wrist-worn activity meter. Theactivity meter 7250 includes a housing 7251, the band 7203, the buckle7204, and the like. Furthermore, the housing 7251 incorporates awireless communication device, a pulse sensor, an acceleration sensor, atemperature sensor, and the like. The activity meter 7250 has a functionof acquiring data such as pulse variation and the amount of activity ofthe user with the pulse sensor and the acceleration sensor and sendingthe data to an external portable information terminal by the wirelesscommunication device. Furthermore, the activity meter 7250 may have afunction of measuring calorie consumption and calorie intake of theuser, a function of measuring the number of steps taken, a function ofmeasuring a sleeping condition, or the like. Note that the activitymeter 7250 may be provided with a display portion for displaying dataacquired by the above function.

The activity meter 7250 includes the power storage device of oneembodiment of the present invention. For example, the power storagedevice 7104 illustrated in FIG. 24E that is in the state of being curvedcan be provided in the housing 7201. Alternatively, the power storagedevice 7104 illustrated in FIG. 24E can be provided in the band 7203such that it can be curved.

FIG. 25B illustrates an example of an armband display device. A displaydevice 7300 includes a display portion 7304 and the power storage deviceof one embodiment of the present invention. The display device 7300 caninclude a touch sensor in the display portion 7304 and can serve as aportable information terminal.

The display surface of the display portion 7304 is bent, and images canbe displayed on the bent display surface. A display state of the displaydevice 7300 can be changed by, for example, near field communication,which is a communication method based on an existing communicationstandard.

The display device 7300 includes an input output terminal, and data canbe directly transmitted to and received from another informationterminal via a connector. In addition, charging via the input outputterminal is possible. Note that the charging operation may be performedby wireless power feeding without using the input output terminal.

FIG. 25C illustrates an example of a glasses-type display device. Adisplay device 7350 includes lenses 7351, a frame 7352, and the like.Furthermore, a projection portion (not illustrated) that projects animage or video on the lenses 7351 is provided in the frame 7352 or incontact with the frame 7352. The display device 7350 has a function ofdisplaying an image 7351A on the entire lenses 7351 in the direction inwhich the user can see the image 7351A. Alternatively, the displaydevice 7350 has a function of displaying an image 7351B on part of thelenses 7351 in the direction in which the user can see the image 7351B.

The display device 7350 includes the power storage device of oneembodiment of the present invention. FIG. 25D is an enlarged view of anedge portion 7355 of the frame 7352. The edge portion 7355 can be formedusing a rubber material such as fluorine rubber or silicone rubber. Thepower storage device 7360 of one embodiment of the present invention isembedded in the edge portion 7355, and the positive electrode lead 7361and the negative electrode lead 7362 protrude from the edge portion7355. The positive electrode lead 7361 and the negative electrode lead7362 are electrically connected to a wiring provided in the frame 7352and connected to a projection portion or the like. Note that the edgeportion 7355 can be formed so as to incorporate the power storage device7360 as in Embodiment 2.

The edge portion 7355 and the power storage device 7360 haveflexibility. Thus, the display device 7350 can be worn so as to be inclose contact with the user along with the shape of the head of theuser.

FIGS. 26A and 26B illustrate an example of a tablet terminal that can befolded in half. A tablet terminal 9600 illustrated in FIGS. 26A and 26Bincludes a pair of housings 9630, a movable portion 9640 connecting thepair of housings 9630, a display portion 9631 a, a display portion 9631b, a display mode changing switch 9626, a power switch 9627, a powersaving mode changing switch 9625, a fastener 9629, and an operationswitch 9628. FIG. 26A illustrates the tablet terminal 9600 that isopened, and FIG. 26B illustrates the tablet terminal 9600 that isclosed.

The tablet terminal 9600 includes a power storage unit 9635 inside thehousings 9630. The power storage unit 9635 is provided across thehousings 9630, passing through the movable portion 9640.

Part of the display portion 9631 a can be a touch panel region 9632 a,and data can be input by touching operation keys 9638 that aredisplayed. Note that FIG. 26A shows, as an example, that half of thearea of the display portion 9631 a has only a display function and theother half of the area has a touch panel function. However, thestructure of the display portion 9631 a is not limited to this, and allthe area of the display portion 9631 a may have a touch panel function.For example, all the area of the display portion 9631 a can display akeyboard and serve as a touch panel while the display portion 9631 b canbe used as a display screen.

As in the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a keyboard display switching button9639 displayed on the touch panel is touched with a finger, a stylus, orthe like, a keyboard can be displayed on the display portion 9631 b.

Touch input can be performed in the touch panel region 9632 a and thetouch panel region 9632 b at the same time.

The display mode changing switch 9626 allows switching between alandscape mode and a portrait mode, color display and black-and-whitedisplay, and the like. The power saving mode changing switch 9625 cancontrol display luminance in accordance with the amount of externallight in use of the tablet terminal 9600, which is measured with anoptical sensor incorporated in the tablet terminal 9600. In addition tothe optical sensor, other detecting devices such as sensors fordetermining inclination, such as a gyroscope or an acceleration sensor,may be incorporated in the tablet terminal.

Although the display portion 9631 a and the display portion 9631 b havethe same area in FIG. 26A, one embodiment of the present invention isnot limited to this example. The display portion 9631 a and the displayportion 9631 b may have different areas or different display quality.For example, one of the display portions 9631 a and 9631 b may displayhigher definition images than the other.

The tablet terminal is closed in FIG. 26B. The tablet terminal includesthe housings 9630, a solar cell 9633, and a charge and discharge controlcircuit 9634 including a DC-DC converter 9636. The power storage deviceof one embodiment of the present invention is used as the power storageunit 9635.

The tablet terminal 9600 can be folded such that the housings 9630overlap with each other when not in use. Thus, the display portions 9631a and 9631 b can be protected, which increases the durability of thetablet terminal 9600. In addition, the power storage unit 9635 of oneembodiment of the present invention has flexibility and can berepeatedly bent without a significant decrease in charge and dischargecapacity. Thus, a highly reliable tablet terminal can be provided.

The tablet terminal illustrated in FIGS. 26A and 26B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image) on the display portion, a function ofdisplaying a calendar, a date, or the time on the display portion, atouch-input function of operating or editing data displayed on thedisplay portion by touch input, a function of controlling processing byvarious kinds of software (programs), and the like.

The solar cell 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processing portion, and the like. Note that the solarcell 9633 can be provided on one surface or opposite surfaces of thehousing 9630 and the power storage unit 9635 can be charged efficiently.The use of a lithium-ion battery as the power storage unit 9635 bringsan advantage such as reduction in size.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 26B will be described with reference to a blockdiagram in FIG. 26C. The solar cell 9633, the power storage unit 9635,the DC-DC converter 9636, a converter 9637, switches SW1 to SW3, and thedisplay portion 9631 are illustrated in FIG. 26C, and the power storageunit 9635, the DC-DC converter 9636, the converter 9637, and theswitches SW1 to SW3 correspond to the charge and discharge controlcircuit 9634 in FIG. 26B.

First, an example of operation when electric power is generated by thesolar cell 9633 using external light will be described. The voltage ofelectric power generated by the solar cell is raised or lowered by theDC-DC converter 9636 to a voltage for charging the power storage unit9635. When the display portion 9631 is operated with the electric powerfrom the solar cell 9633, the switch SW1 is turned on and the voltage ofthe electric power is raised or lowered by the converter 9637 to avoltage needed for operating the display portion 9631. When display onthe display portion 9631 is not performed, the switch SW1 is turned offand the switch SW2 is turned on, so that the power storage unit 9635 canbe charged.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, one embodiment of the present invention isnot limited to this example. The power storage unit 9635 may be chargedusing another power generation means such as a piezoelectric element ora thermoelectric conversion element (Peltier element). For example, thepower storage unit 9635 may be charged with a non-contact powertransmission module capable of performing charging by transmitting andreceiving electric power wirelessly (without contact), or any of theother charge means used in combination.

FIG. 27 illustrates other examples of electronic devices. In FIG. 27, adisplay device 8000 is an example of an electronic device including apower storage device 8004 of one embodiment of the present invention.Specifically, the display device 8000 corresponds to a display devicefor TV broadcast reception and includes a housing 8001, a displayportion 8002, speaker portions 8003, and the power storage device 8004.The power storage device 8004 of one embodiment of the present inventionis provided in the housing 8001. The display device 8000 can receiveelectric power from a commercial power supply. Alternatively, thedisplay device 8000 can use electric power stored in the power storagedevice 8004. Thus, the display device 8000 can be operated with the useof the power storage device 8004 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 8002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like besides TV broadcast reception.

In FIG. 27, an installation lighting device 8100 is an example of anelectronic device including a power storage device 8103 of oneembodiment of the present invention. Specifically, the lighting device8100 includes a housing 8101, a light source 8102, and the power storagedevice 8103. Although FIG. 27 illustrates the case where the powerstorage device 8103 is provided in a ceiling 8104 on which the housing8101 and the light source 8102 are installed, the power storage device8103 may be provided in the housing 8101. The lighting device 8100 canreceive electric power from a commercial power supply. Alternatively,the lighting device 8100 can use electric power stored in the powerstorage device 8103. Thus, the lighting device 8100 can be operated withthe use of power storage device 8103 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

Note that although the installation lighting device 8100 provided in theceiling 8104 is illustrated in FIG. 27 as an example, the power storagedevice of one embodiment of the present invention can be used in aninstallation lighting device provided in, for example, a wall 8105, afloor 8106, a window 8107, or the like other than the ceiling 8104.Alternatively, the power storage device of one embodiment of the presentinvention can be used in a tabletop lighting device or the like.

As the light source 8102, an artificial light source which emits lightartificially by using electric power can be used. Specifically, anincandescent lamp, a discharge lamp such as a fluorescent lamp, andlight-emitting elements such as an LED and an organic EL element aregiven as examples of the artificial light source.

In FIG. 27, an air conditioner including an indoor unit 8200 and anoutdoor unit 8204 is an example of an electronic device including apower storage device 8203 of one embodiment of the present invention.Specifically, the indoor unit 8200 includes a housing 8201, an airoutlet 8202, and the power storage device 8203. Although FIG. 27illustrates the case where the power storage device 8203 is provided inthe indoor unit 8200, the power storage device 8203 may be provided inthe outdoor unit 8204. Alternatively, the power storage devices 8203 maybe provided in both the indoor unit 8200 and the outdoor unit 8204. Theair conditioner can receive electric power from a commercial powersupply. Alternatively, the air conditioner can use electric power storedin the power storage device 8203. Particularly in the case where thepower storage devices 8203 are provided in both the indoor unit 8200 andthe outdoor unit 8204, the air conditioner can be operated with the useof the power storage device 8203 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 27 as an example, thepower storage device of one embodiment of the present invention can beused in an air conditioner in which the functions of an indoor unit andan outdoor unit are integrated in one housing.

In FIG. 27, an electric refrigerator-freezer 8300 is an example of anelectronic device including a power storage device 8304 of oneembodiment of the present invention. Specifically, the electricrefrigerator-freezer 8300 includes a housing 8301, a door for arefrigerator 8302, a door for a freezer 8303, and the power storagedevice 8304. The power storage device 8304 is provided in the housing8301 in FIG. 27. The electric refrigerator-freezer 8300 can receiveelectric power from a commercial power supply. Alternatively, theelectric refrigerator-freezer 8300 can use electric power stored in thepower storage device 8304. Thus, the electric refrigerator-freezer 8300can be operated with the use of the power storage device 8304 of oneembodiment of the present invention as an uninterruptible power supplyeven when electric power cannot be supplied from a commercial powersupply due to power failure or the like.

Note that a high-frequency heating apparatus such as a microwave ovenand an electronic device such as an electric rice cooker require highpower in a short time. The tripping of a breaker of a commercial powersupply in use of an electronic device can be prevented by using thepower storage device of one embodiment of the present invention as anauxiliary power supply for supplying electric power which cannot besupplied enough by a commercial power supply.

In addition, in a time period when electronic devices are not used,particularly when the proportion of the amount of electric power whichis actually used to the total amount of electric power which can besupplied from a commercial power supply source (such a proportionreferred to as a usage rate of electric power) is low, electric powercan be stored in the power storage device, whereby the usage rate ofelectric power can be reduced in a time period when the electronicdevices are used. For example, in the case of the electricrefrigerator-freezer 8300, electric power can be stored in the powerstorage device 8304 in night time when the temperature is low and thedoor for a refrigerator 8302 and the door for a freezer 8303 are notoften opened or closed. On the other hand, in daytime when thetemperature is high and the door for a refrigerator 8302 and the doorfor a freezer 8303 are frequently opened and closed, the power storagedevice 8304 is used as an auxiliary power supply; thus, the usage rateof electric power in daytime can be reduced.

Furthermore, the power storage device of one embodiment of the presentinvention can be provided in a vehicle.

The use of power storage devices in vehicles enables production ofnext-generation clean energy vehicles such as hybrid electric vehicles(HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles(PHEVs).

FIGS. 28A and 28B each illustrate an example of a vehicle using oneembodiment of the present invention. An automobile 8400 illustrated inFIG. 28A is an electric vehicle that runs on the power of an electricmotor. Alternatively, the automobile 8400 is a hybrid electric vehiclecapable of driving appropriately using either the electric motor or theengine. One embodiment of the present invention can provide ahigh-mileage vehicle. The automobile 8400 includes the power storagedevice. The power storage device is used not only for driving theelectric motor, but also for supplying electric power to alight-emitting device such as a headlight 8401 or a room light (notillustrated).

The power storage device can also supply electric power to a displaydevice of a speedometer, a tachometer, or the like included in theautomobile 8400. Furthermore, the power storage device can supplyelectric power to a semiconductor device included in the automobile8400, such as a navigation system.

FIG. 28B illustrates an automobile 8500 including the power storagedevice. The automobile 8500 can be charged when the power storage deviceis supplied with electric power through external charging equipment by aplug-in system, a contactless power feeding system, or the like. In FIG.28B, a power storage device included in the automobile 8500 is chargedwith the use of a ground-based charging apparatus 8021 through a cable8022. In charging, a given method such as CHAdeMO (registered trademark)or Combined Charging System may be employed as a charging method, thestandard of a connector, or the like as appropriate. The ground-basedcharging apparatus 8021 may be a charging station provided in a commercefacility or a power source in a house. For example, with the use of aplug-in technique, the power storage device included in the automobile8500 can be charged by being supplied with electric power from outside.The charging can be performed by converting AC electric power into DCelectric power through a converter such as an AC-DC converter.

Furthermore, although not illustrated, the vehicle may include a powerreceiving device so that it can be charged by being supplied withelectric power from an above-ground power transmitting device in acontactless manner. In the case of the contactless power feeding system,by fitting a power transmitting device in a road or an exterior wall,charging can be performed not only when the electric vehicle is stoppedbut also when driven. In addition, the contactless power feeding systemmay be utilized to perform transmission and reception of electric powerbetween vehicles. Furthermore, a solar cell may be provided in theexterior of the automobile to charge the power storage device when theautomobile stops or moves. To supply electric power in such acontactless manner, an electromagnetic induction method or a magneticresonance method can be used.

According to one embodiment of the present invention, the power storagedevice can have improved cycle characteristics and reliability.Furthermore, according to one embodiment of the present invention, thepower storage device itself can be made more compact and lightweight asa result of improved characteristics of the power storage device. Thecompact and lightweight power storage device contributes to a reductionin the weight of a vehicle, and thus increases the driving distance.Furthermore, the power storage device included in the vehicle can beused as a power source for supplying electric power to products otherthan the vehicle. In such a case, the use of a commercial power sourcecan be avoided at peak time of electric power demand.

This embodiment can be combined with any of the other embodiments asappropriate.

EXAMPLE 1

In this example, evaluation results of the characteristics of the powerstorage device of one embodiment of the present invention that wasfabricated will be described.

In this example, the power storage device 500 illustrated in FIG. 1A wasfabricated.

In this example, the following 8 samples formed using one embodiment ofthe present invention were used in total: Samples A1, A2, B1, B2, C1,C2, D1, and D2.

The samples fabricated in this example each include one positiveelectrode in which a positive electrode active material layer isprovided on one surface of a positive electrode current collector andone negative electrode in which a negative electrode active materiallayer is provided on one surface of a negative electrode currentcollector. In other words, the samples in this example each include onepositive electrode active material layer and one negative electrodeactive material layer.

First, methods for fabricating the electrodes will be described.

[Fabricating Method for Negative Electrode]

The same fabricating method was used to form the negative electrodes ofall the samples in this example.

Spherical natural graphite having a specific surface area of 6.3 m²/gand an average particle size of 15 μm (CGB-15 manufactured by NipponGraphite Industries, Co., Ltd.) was used as a negative electrode activematerial. For a binder, sodium carboxymethyl cellulose (CMC-Na) and SBRwere used. The polymerization degree of CMC-Na that was used was higherthan or equal to 600 and lower than or equal to 800, and the viscosityof a 1 wt % CMC-Na aqueous solution was in the range from 300 mPa·s to500 mPa·s. The compounding ratio of graphite:CMC-Na:SBR was set to97:1:1.5 (wt %).

First, CMC-Na powder and an active material were mixed and then kneadedwith a mixer, so that a first mixture was obtained.

Subsequently, a small amount of water was added to the first mixture andkneading was performed, so that a second mixture was obtained. Here,“kneading” means “mixing something with a high viscosity”.

Then, water was further added and the mixture was kneaded with a mixer,so that a third mixture was obtained.

Then, a 50 wt % SBR aqueous dispersion liquid was added to the thirdmixture, and mixing was performed with a mixer. After that, the obtainedmixture was degassed under a reduced pressure, so that a slurry wasobtained.

Subsequently, the slurry was applied to a negative electrode currentcollector with the use of a continuous coater. An 18-μm-thick rolledcopper foil was used as the negative electrode current collector. Thecoating speed was set to 0.75 m/min.

Then, the solvent in the slurry applied to the negative electrodecurrent collector was vaporized in a drying furnace. Vaporizationtreatment was performed at 50° C. in an air atmosphere for 120 secondsand then further performed at 80° C. in the air atmosphere for 120seconds. After that, further vaporization treatment was performed at100° C. under a reduced pressure (−100 KPa) for 10 hours.

Through the above steps, the negative electrode active material layerwas formed on one surface of the negative electrode current collector,so that the negative electrode was fabricated.

[Fabrication Method for Positive Electrode]

The same fabrication method was used to fabricate the positiveelectrodes of all the samples in this example.

As each positive electrode active material, LiCoO₂ with a specificsurface area of 0.21 m²/g and an average particle size of 10 μm wasused. As each binder, polyvinylidene fluoride (PVdF) was used. As eachconductive additive, acetylene black was used. The compounding ratio ofLiCoO₂:acetylene black:PVdF was set to 90:5:5 (wt %).

First, acetylene black and PVdF were mixed in a mixer, so that a firstmixture was obtained.

Next, the active material was added to the first mixture, so that asecond mixture was obtained.

After that, a solvent N-methyl-2-pyrrolidone (NMP) was added to thesecond mixture and mixing was performed with a mixer. Through the abovesteps, a slurry was formed.

Then, mixing was performed with a large-sized mixer.

Subsequently, the slurry was applied to a positive electrode currentcollector with the use of a continuous coater. A 20-μm-thick aluminumcurrent collector was used as the positive electrode current collector.The coating speed was set to 0.2 m/min.

Then, the solvent in the slurry applied to the positive electrodecurrent collector was vaporized in a drying furnace. Solventvaporization treatment was performed at 70° C. in an air atmosphere for7.5 minutes and then further performed at 90° C. in the air atmospherefor 7.5 minutes.

After that, heat treatment was performed in a reduced-pressureatmosphere (at a gauge pressure of −100 kPa) at 170° C. for 10 hours.Subsequently, the positive electrode active material layer was pressedby a roll press method so as to be consolidated.

Through the above steps, the positive electrode active material layerwas formed on one surface of the positive electrode current collector,so that the positive electrode was fabricated.

Table 1 lists the averages of the active material loadings, thethicknesses, and the densities of each of the positive electrode activematerial layers and the negative electrode active material layers thatwere formed. The values shown in this specification are the averages ofmeasurement values of each of the electrodes used in fabricating thesamples. Note that when the active material layers were formed onopposite surfaces of the current collector, the values are the averagesof the active material loadings, the thicknesses, and the densities ofthe active material layer on one surface of the current collector.

TABLE 1 Sample Sample Sample Sample A1 A2 B1 B2 Positive Load 19.8 19.819.8 19.8 electrode (mg/cm²) Thickness 82 80 79 78 (μm) Density 2.422.48 2.50 2.54 (g/cc) Negative Load 10.2 10.2 10.3 10.3 electrode(mg/cm²) Thickness 108 107 111 110 (μm) Density 0.94 0.95 0.92 0.93(g/cc) Sample Sample Sample Sample C1 C2 D1 D2 Positive Load 19.8 19.819.0 19.9 electrode (mg/cm²) Thickness 81 81 84 75 (μm) Density 2.402.40 2.26 2.65 (g/cc) Negative Load 10.2 10.2 10.1 10.4 electrode(mg/cm²) Thickness 108 107 108 111 (μm) Density 0.94 0.95 0.93 0.93(g/cc)

In electrolytic solutions, a solvent in which EC and PC are mixed at avolume ratio of 1:1 was used, and various kinds of solutes and additiveswere used as in Table 2. In each of Samples A1 and A2 (hereinafter, thecomposition of the electrolytic solutions of these samples is referredto as Condition A), 1 mol/l of LiTFSA was used as a solute of theelectrolytic solution, and 1 wt % of VC and 2 wt % of LiFSA were used asadditives. In each of Samples B1 and B2 (hereinafter, the composition ofthe electrolytic solutions of these samples is referred to as ConditionB), 1 mol/l of LiFSA was used as a solute of the electrolytic solution,and 1 wt % of VC was used as an additive. In each of Samples C1 and C2(hereinafter, the composition of the electrolytic solutions of thesesamples is referred to as Condition C), 1 mol/l of LiBETA was used as asolute of the electrolytic solution, and 1 wt % of VC was used as anadditive. In each of Samples D1 and D2 (hereinafter, the composition ofthe electrolytic solutions of these samples is referred to as ConditionD), 1 mol/l of LiBETA was used as a solute of the electrolytic solution,and 1 wt % of PS was used as an additive. In each sample of Condition A,LiFSA was used as the additive, whereas in each sample of Condition B,LiFSA was used as the solute.

TABLE 2 Sample Sample Sample Sample A1, A2 B1, B2 C1, C2 D1, D2Electrolytic Solvent EC:PC = 1:1 (v/v) solution Solute LiTFSA LiFSALiBETA LiBETA 1 mol/l 1 mol/1 1 mol/l 1 mol/l Additive 1 VC 1 wt % PS 1wt % Additive 2 LiFSA — 2 wt %

As each separator, a stack of two 46-μm-thick separators usingpolyphenylene sulfide (hereinafter also referred to as PPS separators)was used.

As an exterior body, an aluminum film with opposite surfaces coveredwith a resin layer was used.

Next, fabrication methods for the samples will be described.

First, a positive electrode, a negative electrode, and a separator werecut. The size of the positive electrode is 20.49 cm², and the size ofthe negative electrode is 23.84 cm².

Then, the positive electrode active material and the negative electrodeactive material in tab regions were removed to expose the currentcollectors.

After that, the positive electrode and the negative electrode werestacked with the separator therebetween. At this time, the positiveelectrode and the negative electrode were stacked such that the positiveelectrode active material layer and the negative electrode activematerial layer faced each other.

Then, leads were attached to the positive electrode and the negativeelectrode by ultrasonic welding.

Then, facing parts of two of four sides of the exterior body were bondedto each other by heating.

After that, sealing layers provided for the leads were positioned so asto overlap with a sealing layer of the exterior body, and bonding wasperformed by heating. At this time, facing parts of a side of theexterior body except a side used for introduction of an electrolyticsolution were bonded to each other.

Next, heat treatment for drying the exterior body and the positiveelectrode, the separator, and the negative electrode wrapped by theexterior body was performed in a reduced-pressure atmosphere (at a gaugepressure of −100 kPa) at 80° C. for 10 hours.

Subsequently, in an argon gas atmosphere, an approximately 600 μl ofelectrolytic solution was introduced from one side of the exterior bodythat was not sealed. After that, the one side of the exterior body wassealed by heating in a reduced-pressure atmosphere (at a gauge pressureof −60 kPa). Through the above steps, each thin storage battery wasfabricated.

Next, heat treatment was performed on Samples A2, B2, C2, and D2.Assuming that each sample and fluorine rubber are integrally formed asin Embodiment 2, heat treatment was performed in an atmospheric pressureatmosphere at 170° C. for 15 minutes. Specifically, the temperature of athermostatic bath was raised to approximately 170° C., each sample wasput in the thermostatic bath, and after 15 minutes, the sample was takenout. Expansion accompanying the heat treatment did not occur in theexterior body of each sample.

Through the above steps, the samples were fabricated.

Next, the charge and discharge characteristics at 25° C. of the samplesin this example were measured. The measurement was performed with acharge-discharge measuring instrument (produced by TOYO SYSTEM Co.,LTD.). Constant current-constant voltage charging was performed untilthe voltage reached an upper voltage limit of 4.3 V, and constantvoltage discharging was performed until the voltage reached a lowervoltage limit of 2.5 V. The charging and discharging were performed at arate of 0.1 C, and a 10-minute break was taken after the charging. Notethat two charge and discharge cycles were performed.

Here, a charge rate and a discharge rate will be described. A chargerate of 1 C means a current value at which a cell with a capacity of X(Ah) is charged at a constant current such that charging is terminatedin exactly 1 hour. When 1 C=I(A), a charge rate of 0.2 C means I/5 (A),i.e., a current value at which charging is terminated in exactly 5hours. Similarly, a discharge rate of 1 C means a current value at whicha cell with a capacity of X (Ah) is discharged at a constant currentsuch that discharging is terminated in exactly 1 hour. A discharge rateof 0.2 C means I/5 (A), i.e., a current value at which discharging isterminated in exactly 5 hours. Note that the rates were calculated using170 mAh/g, which is capacity obtained when the upper charging voltagelimit of LiCoO₂ serving as the positive electrode active material, is4.3 V, as a reference.

FIG. 29A shows charge and discharge curves of Sample A1. FIG. 29B showscharge and discharge curves of Sample A2. FIG. 29C shows charge anddischarge curves of Sample B1. FIG. 29D shows charge and dischargecurves of Sample B2. FIG. 30A shows charge and discharge curves ofSample C1. FIG. 30B shows charge and discharge curves of Sample C2. FIG.30C shows charge and discharge curves of Sample D1. FIG. 30D showscharge and discharge curves of Sample D2. In FIGS. 29A to 29D and FIGS.30A to 30D, the horizontal axis represents capacity (mAh/g), and thevertical axis represents voltage (V).

As shown in FIGS. 29A and 29C, abnormal conditions occurred in the firstcharging of Samples A1, A2, B1, and B2, and the characteristics ofSamples A1, A2, B1, and B2 were noticeably degraded in the firstdischarging. In the second discharging, the characteristics were furtherdegraded. This suggests that LiTFSA and LiFSA, which are solutes of theelectrolytic solutions, corrode aluminum of the positive electrodecurrent collectors in the state where the potentials of the positiveelectrodes are high. Similar abnormal conditions occurred in the samplesof Conditions A and B subjected to heat treatment (see FIGS. 29B and29D).

In contrast, as shown in FIGS. 30A and 30C, the first charging and thesecond charging were normally performed in Samples C1, C2, D1, and D2,and they have favorable charge and discharge characteristics. Theseresults indicate that even in charging and discharging at a chargingvoltage of 4.3 V, corrosion of the positive electrode current collectorsin the power storage devices using LiBETA as the solutes was inhibitedand thus the power storage devices stably operated. As shown in FIGS.30B and 30D, decreases in the capacities of the samples of Conditions Cand D subjected to heat treatment are small, and the samples have normalcharge and discharge characteristics and high heat resistance. Table 3shows the discharge capacity retention rates of the samples ofConditions C and D that are subjected to heat treatment. The dischargecapacity retention rates were calculated using respective dischargecapacities of the samples in the second cycle.

TABLE 3 Capacity Capacity Capacity obtained obtained retention withoutheat with heat ratio obtained treatment treatment by heating (mAh/g)(mAh/g) (%) Condition C 126.7 (C1) 111.9 (C2) 88.3 Condition D 129.0(D1) 118.6 (D2) 91.9

FIGS. 30B and 30D and Table 3 indicate that the power storage devicesusing the electrolytic solution of Condition D have the highest heatresistance and the highest battery capacity.

EXAMPLE 2

In this example, results obtained by performing experiments to examinethe state of a surface of spherical natural graphite used for thenegative electrode active material included in the power storage deviceof one embodiment of the present invention will be described.

[Cross-Sectional TEM Observation]

Spherical natural graphite powder was sliced by a focused ion beam (FIB)method and taken out as a sample. The sample was observed with across-sectional transmission electron microscope (TEM) (H-9000NARmanufactured by Hitachi High-Technologies Corporation) at anacceleration voltage of 200 kV. FIGS. 31A to 31C show obtained TEMimages. FIGS. 31B and 31C are enlarged TEM images showing a region 900including the vicinity of an edge plane and a region 901 including thevicinity of a basal plane in FIG. 31A, respectively. Note that a minuteedge presumably exists even in a plane seen as the basal plane in thecross-sectional TEM observation.

The spherical natural graphite has a structure in which a graphite layeris folded (see FIG. 31A). As shown in FIGS. 31B and 31C, a coating layer912 having lower crystallinity than graphite layers 911 arrangedregularly is located outward from the graphite layers 911 (as theoutermost surface layer of the spherical natural graphite) both in thevicinity of the edge plane and in the vicinity of the basal plane of thespherical natural graphite.

[Raman Spectroscopy]

Next, analysis results of Raman spectra by Raman spectroscopy will bedescribed. For the analysis, two-point measurement was performed onspherical natural graphite powder with the use of a Raman microscope(LabRAM manufactured by HORIBA, Ltd.). Note that the wavelength of laserlight used for the Raman analysis is 532 nm.

FIG. 32 shows analysis results of Raman spectra of the spherical naturalgraphite powder. In FIG. 32, the D band (the peak at around 1360 cm⁻¹ ofthe Raman spectra) showing crystallinity disorder of graphite is clearlyobserved. Table 4 lists the intensity ratios (R values) of the D band tothe G band (the peak at around 1580 cm⁻¹ of the Raman spectra). Sincethe D band is clearly observed, the R values are not small,specifically, 0.28 and 0.38. The R values, which are not small, may havea relation with the fact that the layer having reduced crystallinity onthe surface of the spherical natural graphite is observed as shown inFIGS. 31B and 31C.

TABLE 4 Rvalue (D/G ratio) Point 1 0.38 Point 2 0.28

Such a layer having low crystallinity on the outermost surface of agraphite particle, which can be observed with a TEM or by Ramanspectroscopy as in this example, may be able to inhibit PC intercalationbetween graphite layers.

EXAMPLE 3

In this example, evaluation results of the characteristics of the powerstorage device of one embodiment of the present invention that wasfabricated will be described.

In this example, the power storage device 500 illustrated in FIG. 1A wasfabricated.

In this example, the following two samples formed using one embodimentof the present invention were used in total: Samples E1 and E2.

The samples fabricated in this example each include one positiveelectrode in which a positive electrode active material layer isprovided on one surface of a positive electrode current collector andone negative electrode in which a negative electrode active materiallayer is provided on one surface of a negative electrode currentcollector. In other words, the samples in this example each include onepositive electrode active material layer and one negative electrodeactive material layer.

First, methods for fabricating the electrodes will be described.

[Fabricating Method for Negative Electrode]

The same fabricating method was used to form the negative electrodes ofall the samples in this example.

Spherical natural graphite having a specific surface area of 6.3 m²/gand an average particle size of 15 μm (CGB-15 manufactured by NipponGraphite Industries, Co., Ltd.) was used as a negative electrode activematerial. For a binder, sodium carboxymethyl cellulose (CMC-Na) and SBRwere used. The polymerization degree of CMC-Na that was used was higherthan or equal to 600 and lower than or equal to 800, and the viscosityof a 1 wt % CMC-Na aqueous solution was in the range from 300 mPa·s to500 mPa·s. The compounding ratio of graphite:CMC-Na:SBR was set to97:1.5:1.5 (wt %).

First, CMC-Na powder and an active material were mixed and then kneadedwith a mixer, so that a first mixture was obtained.

Subsequently, a small amount of water was added to the first mixture andkneading was performed, so that a second mixture was obtained. Here,“kneading” means “mixing something with a high viscosity”.

Then, water was further added and the mixture was kneaded with a mixer,so that a third mixture was obtained.

Then, a 50 wt % SBR aqueous dispersion liquid was added to the thirdmixture, and mixing was performed with a mixer. After that, the obtainedmixture was degassed under a reduced pressure, so that a slurry wasobtained.

Subsequently, the slurry was applied to a negative electrode currentcollector with the use of a continuous coater. An 18-μm-thick rolledcopper foil was used as the negative electrode current collector. Thecoating speed was set to 0.75 m/min.

Then, the solvent in the slurry applied to the negative electrodecurrent collector was vaporized in a drying furnace. Vaporizationtreatment was performed at 50° C. in an air atmosphere for 120 secondsand then further performed at 80° C. in the air atmosphere for 120seconds. After that, further vaporization treatment was performed at100° C. under a reduced pressure (−100 KPa) for 10 hours.

Through the above steps, the negative electrode active material layerwas formed on opposite surfaces of the negative electrode currentcollector, so that the negative electrode was fabricated.

[Fabrication Method for Positive Electrode]

The same fabrication method was used to fabricate the positiveelectrodes of all the samples in this example.

As each positive electrode active material, LiCoO₂ with an averageparticle size of 6 μm was used. As each binder, polyvinylidene fluoride(PVdF) was used. As each conductive additive, acetylene black was used.The compounding ratio of LiCoO₂:acetylene black:PVdF was set to 95:3:2(wt %).

First, acetylene black and PVdF were mixed in a mixer, so that a firstmixture was obtained.

Next, the active material was added to the first mixture, so that asecond mixture was obtained.

After that, a solvent N-methyl-2-pyrrolidone (NMP) was added to thesecond mixture and mixing was performed with a mixer. Through the abovesteps, a slurry was formed.

Then, mixing was performed with a large-sized mixer.

Subsequently, the slurry was applied to a positive electrode currentcollector with the use of a continuous coater. A 20-μm-thick aluminumcurrent collector was used as the positive electrode current collector.The coating speed was set to 0.2 m/min.

Then, the solvent in the slurry applied to the positive electrodecurrent collector was vaporized in a drying furnace. Solventvaporization treatment was performed at 70° C. in an air atmosphere for7.5 minutes and then further performed at 90° C. in the air atmospherefor 7.5 minutes.

After that, heat treatment was performed in a reduced-pressureatmosphere (at a gauge pressure of −100 kPa) at 170° C. for 10 hours.Subsequently, the positive electrode active material layer was pressedby a roll press method so as to be consolidated.

Through the above steps, the positive electrode active material layerwas formed on one surface of the positive electrode current collector,so that the positive electrode was fabricated.

Table 5 lists the averages of the active material loadings, thethicknesses, and the densities of each of the positive electrode activematerial layers and the negative electrode active material layers thatwere formed. The values shown in this specification are the averages ofmeasurement values of each of the electrodes used in fabricating thesamples. Note that when the active material layers were formed onopposite surfaces of the current collector, the values are the averagesof the active material loadings, the thicknesses, and the densities ofthe active material layer on one surface of the current collector.

TABLE 5 Sample Sample E1 E2 Positive Load 19.1 21.4 electrode (mg/cm²)Thickness 61 69 (μm) Density 3.14 3.10 (g/cc) Negative Load 10.3 10.4electrode (mg/cm²) Thickness 113 113 (μm) Density 0.92 0.92 (g/cc)

In each electrolytic solution, a solvent in which EC and PC are mixed ata volume ratio of 1:1 was used, 1 mol/l of LiBETA was used as a solute,and 1 wt % of PS was used as an additive. Table 6 lists the condition ofthe electrolytic solution. The condition of the electrolytic solutionfor Samples E1 and E2 is similar to Condition D in Example 1.

TABLE 6 Sample E1, E2 Electrolytic Solvent EC:PC = 1:1 (v/v) solutionSolute LiBETA 1 mol/l Additive 1 PS 1 wt % Additive 2 —

As each separator, a stack of two 46-μm-thick separators usingsolvent-spun regenerated cellulosic fiber was used.

As an exterior body, an aluminum film with opposite surfaces coveredwith a resin layer was used.

Next, fabrication methods for the samples will be described.

First, a positive electrode, a negative electrode, and a separator werecut. The size of the positive electrode is 20.49 cm², and the size ofthe negative electrode is 23.84 cm².

Then, the positive electrode active material and the negative electrodeactive material in tab regions were removed to expose the currentcollectors.

After that, the positive electrode and the negative electrode werestacked with the separator therebetween. At this time, the positiveelectrode and the negative electrode were stacked such that the positiveelectrode active material layer and the negative electrode activematerial layer faced each other.

Then, leads were attached to the positive electrode and the negativeelectrode by ultrasonic welding.

Then, the stack of the positive electrode and the negative electrode waswrapped in a sheet using polyphenylene sulfide in order to prevent thepositive electrode or the negative electrode from coming in contact withan aluminum layer of the exterior body when the aluminum layer isexposed by heat treatment performed on the battery; accordingly, a shortcircuit can be prevented.

Then, facing parts of two of four sides of the exterior body were bondedto each other by heating.

After that, sealing layers provided for the leads were positioned so asto overlap with a sealing layer of the exterior body, and bonding wasperformed by heating. At this time, facing parts of a side of theexterior body except a side used for introduction of an electrolyticsolution were bonded to each other.

Next, heat treatment for drying the exterior body and the positiveelectrode, the separator, and the negative electrode wrapped by theexterior body was performed in a reduced-pressure atmosphere (at a gaugepressure of −100 kPa) at 80° C. for 10 hours.

Subsequently, in an argon gas atmosphere, an approximately 600 μl ofelectrolytic solution was introduced from one side of the exterior bodythat was not sealed. After that, the one side of the exterior body wassealed by heating in a reduced-pressure atmosphere (at a gauge pressureof −100 kPa). Through the above steps, each thin power storage devicewas fabricated.

Next, heat treatment was performed on Sample E2. Assuming that thesample and fluorine rubber are integrally formed as in Embodiment 2,heat treatment was performed in an atmospheric pressure atmosphere at170° C. for 15 minutes. Specifically, the temperature of a thermostaticbath was raised to approximately 170° C., the sample was put in thethermostatic bath, and after 15 minutes, the sample was taken out.Expansion accompanying the heat treatment did not occur in the exteriorbody of the sample.

Through the above steps, the samples were fabricated.

Next, the charge and discharge characteristics at 25° C. of the samplesin this example were measured. The measurement was performed with acharge-discharge measuring instrument (produced by TOYO SYSTEM Co.,LTD.). Constant current-constant voltage charging was performed untilthe voltage reached an upper voltage limit of 4.3 V, and constantvoltage discharging was performed until the voltage reached a lowervoltage limit of 2.5 V. The charging and discharging were performed at arate of 0.1 C, and a 10-minute break was taken after the charging. Notethat two charge and discharge cycles were performed.

FIG. 33A shows charge and discharge curves of Sample E1. FIG. 33B showscharge and discharge curves of Sample E2. In FIGS. 33A and 33B, thehorizontal axis represents capacity (mAh/g), and the vertical axisrepresents voltage (V).

As shown in FIGS. 33A and 33B, the first charging and the secondcharging were normally performed in Samples E1 and E2, and they havefavorable charge and discharge characteristics. Furthermore, the resultsshown in FIG. 33B indicate that a decrease in the capacity of even thesample subjected to heat treatment is small, and the sample has normalcharge and discharge characteristics and high heat resistance.

This application is based on Japanese Patent Application serial no.2015-240755 filed with Japan Patent Office on Dec. 10, 2015, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A power storage device comprising: a positiveelectrode; a negative electrode; a separator; an electrolytic solution;and an exterior body, wherein the positive electrode comprises apositive electrode active material layer and a positive electrodecurrent collector, wherein the negative electrode comprises a negativeelectrode active material layer and a negative electrode currentcollector, wherein the separator is located between the positiveelectrode and the negative electrode, wherein the separator comprisespolyphenylene sulfide or solvent-spun regenerated cellulosic fiber,wherein the electrolytic solution comprises a solute and two or morekinds of solvents, the electrolytic solution comprising propane sultone,wherein the solute comprises lithium bis(pentafluoroethanesulfonyl)amide(LiBETA), wherein the solvents comprise propylene carbonate and ethylenecarbonate, and wherein the propylene carbonate is the highest proportionin the electrolytic solution except the ethylene carbonate.
 2. The powerstorage device according to claim 1, wherein the negative electrodeactive material layer comprises graphite.
 3. The power storage deviceaccording to claim 2, wherein the negative electrode active materiallayer comprises spherical natural graphite, wherein the sphericalnatural graphite comprises a first region and a second region, whereinthe first region covers the second region, and wherein the first regionhas lower crystallinity than the second region.
 4. The power storagedevice according to claim 1, wherein the positive electrode activematerial layer comprises LiCoO₂.
 5. The power storage device accordingto claim 1, wherein the positive electrode current collector comprisesaluminum or stainless steel.
 6. An electronic device comprising: thepower storage device according to claim 1; a band; a display panel; anda housing, wherein the power storage device comprises a positiveelectrode lead and a negative electrode lead, wherein the positiveelectrode lead is electrically connected to the positive electrode,wherein the negative electrode lead is electrically connected to thenegative electrode, wherein the power storage device is buried in theband, wherein part of the positive electrode lead and part of thenegative electrode lead protrude from the band, wherein the powerstorage device has flexibility, wherein the power storage device iselectrically connected to the display panel, wherein the display panelis incorporated in the housing, wherein the band is connected to thehousing, and wherein the band comprises a rubber material.
 7. Theelectronic device according to claim 6, wherein the rubber material isfluorine rubber or silicone rubber.
 8. The power storage deviceaccording to claim 1, wherein the exterior body is in contact with arubber material.
 9. The power storage device according to claim 8,wherein the rubber material has a shape integrally formed with theexterior body.